Addresses are the part of a plan that stays wrong quietly. A duplicate address works for a week and then does not, and an address in the wrong subnet answers ping from one side only. The configurator compares every address you typed, in the site and on the board, against the networks it is generating.
An error means the result will not work. A warning can be a fine decision. A note is there so you are not surprised later.
The same address twice
gw-01 and sw-01 both use 192.168.10.1. and nas-01 and printer-01 both use 192.168.10.20.
Errors. The first is about two devices the tool configures, the second about two hosts you planned on the board. Whoever boots first wins and the other one loses its address at random moments. Give one of them a different address.
Not an IPv4 address
nas-01: 192.168.1.300 is not a valid IPv4 address.
An error, and nearly always a typo: a digit too many, a comma instead of a full stop, or a subnet mask pasted into an address field. Correct the address on the node.
An address that cannot be a LAN address
The LAN uses 169.254.10.1. That cannot be a LAN address: it is reserved for loopback, link-local, multicast or future use.
An error. Loopback (127.x), link-local (169.254.x), multicast (224.x and above), 0.x and the space kept for future use are not addresses a network can be built on. No setting elsewhere in the configuration makes them work. Pick an address in one of the private ranges instead. The message names where it found it: the LAN, or the VLAN the address belongs to.
An address from a documentation range
VLAN 20 uses 192.0.2.1. That block is reserved for documentation and testing, so it belongs in examples rather than in a real network.
A warning. The blocks 192.0.2.0/24, 198.51.100.0/24 and 203.0.113.0/24 exist for manuals and examples, and 198.18.0.0/15 for benchmarking equipment. They usually mean an address from a tutorial ended up in the plan. Nothing breaks on your own network today, but these ranges are not yours and some equipment treats them specially. Renumber to a private range.
An address outside the private ranges
The LAN uses 172.168.30.1, an address outside the private ranges. If this block is yours or your customer's, it is correct and you can ignore this. If it is not, it belongs to somebody else on the internet and you will not be able to reach that destination from this network. Private is 10.x, 172.16 through 172.31, and 192.168.x; note that 172.168 is not one of them.
Information, not an error, and phrased so you can dismiss it. A browser cannot know who owns a block, and a customer with their own range may well use it inside the building.
The case it is written for is the typo: 172.168 looks private and is not. Anything you number that way keeps working inside the building, while every real server in that block becomes unreachable from this network, because the router believes those addresses are local. Check the first two numbers before you generate.
The shared range 100.64.0.0/10 is deliberately left alone. It is what providers use for CGNAT and what several fleets use internally, so the tool says nothing about it.
The address is outside its own network
nas-01: 192.168.5.20 is outside VLAN 20 (192.168.20.0/24).
A warning. The host is on that VLAN according to the board, but the address belongs to another network, so it will not reach its gateway. It usually means you renumbered the VLAN and not the hosts. Correct the address, or put the host on the VLAN that matches it.
An address the network keeps for itself
nas-01: 192.168.20.0 is the network or broadcast address of VLAN 20. and nas-01: 192.168.20.1 is the router's own address on VLAN 20.
Errors. The first and the last address of a subnet are not usable for a host, and the router has already taken the first usable one as its gateway address. Pick something further into the range.
A fixed address inside the DHCP range
nas-01: 192.168.20.50 is inside the DHCP range of VLAN 20 (192.168.20.10-192.168.20.254). Keep fixed addresses outside it, or the router may hand out the same address.
A note, and worth acting on. It works until the day the router leases that address to a laptop and two machines answer to it. Put fixed addresses below the pool, or move the pool up.
The subnet is too small for DHCP
The LAN subnet is too small for a DHCP pool. and The offbridge subnet is too small for DHCP; set an address on the laptop by hand.
Warnings. A /30 holds two usable addresses, one of which is the router: there is nothing left to hand out. Either make the subnet bigger, or accept it and configure the client by hand, which for an emergency port is a reasonable choice.
No gateway on a device without WAN
No gateway given: updates, NTP and management from other networks will not work.
A warning on a switch or an access point with a fixed management address. Without a default route the device answers inside its own subnet and nowhere else, so time sync, updates and access from another VLAN all fail. Fill in the router address, or let the device get its address over DHCP.
Locations with the same or an invalid address range
Location Branch: 10.300 is not a valid address range. Use the first two numbers, such as 10.2. and Locations Head office and Branch both use the range 10.1.
Errors. A location on the board gets its own range so the tool can number its VLANs apart from the rest. Two locations on the same two numbers produces two identical sets of subnets. Give each location its own first two numbers.
Two locations that end up on the same subnets
Locations Head office and Branch use the same subnets (192.168.10.0/24). Give one of them its own address range, or they cannot talk over a VPN later.
A warning. Separately both sites work. The day you join them with a tunnel, both ends claim the same networks and no route can decide between them. Renumbering afterwards costs an evening, so do it now.
Both ends of a tunnel use the same networks
Tunnel gw-01 to gw-02: both sides use 192.168.10.0/24, so the routes over the tunnel clash. Put the routers in locations with their own address range.
A warning, and the same problem one step further along: here the tunnel already exists. A router cannot route to a network it thinks it is in. Renumber one side, or put the two routers in separate locations on the board so the tool numbers them apart.
The emergency subnet overlaps a real one
The offbridge address 192.168.88.1/24 overlaps LAN (192.168.88.0/24). Pick a subnet of its own.
An error. The offbridge port exists to reach the device when the normal network is broken. If it sits in the same range as that network, the router does not know which interface to answer on and you have lost the way back in. Give the emergency port a range nothing else uses.
The hotspot address differs from the VLAN address
Hotspot interface vlan-guest already has address 192.168.30.1/24 (VLANs); the hotspot address 192.168.50.1/24 is added on top. Make them equal.
A warning. Two addresses on one interface is legal in RouterOS but means clients get a gateway from one of them and the hotspot listens on the other. Set the hotspot address to the address the VLAN already has.
A gateway the tool cannot fill in
Recursive failover: uplink 2 (ether2) gets its gateway from DHCP. Replace the gateway placeholder in the routes by the real gateway address, or use the distance method. and PCC: replace the gateway placeholders in the routes by the real gateway addresses of both uplinks.
Warnings about something you have to finish by hand. These failover methods need a fixed gateway address in the route, and an address that comes from DHCP is only known once the line is up. Read it with /ip route print after connecting and edit the routes, or pick the distance method, which does not need it.
DNS settings that do not match
ISP DNS chosen, but no WAN has "use ISP DNS" enabled., RDNSS advertises the router as DNS, but allow-remote-requests is off. and Adlist only exists from RouterOS 7.15 and has been left out for this target version.
Warnings. In each case two settings contradict each other and the result is a router that hands out a DNS server that answers nothing. Switch on the matching option, or pick fixed DNS servers instead of the provider ones.
A MAC address you made up
The MAC address you entered does not have the locally administered bit set. Such an address can collide with a factory address on another device; use something like 02:... instead.
A note. Addresses you invent yourself belong in the locally administered space, which is what the second bit of the first byte says. Starting with 02: keeps you clear of every factory address in the world.
Related: LAN and DHCP, Multiple devices in one site and Checks about VLANs.