It is usually one of four things: the server has not finished starting, their Minecraft version does not match the server, they are missing the server's modpack, or they are not on the whitelist.

Go through these in order.

1. Is the server actually running?

A server can take a minute or more to start, and a big modpack much longer. Until the console says "Done", nobody can join. If it keeps restarting, the console or crash report will say why.

2. Do the versions match?

Java Edition players need the same Minecraft version as the server, unless the server has a plugin like ViaVersion that lets newer clients join. The error usually says "Outdated client" or "Outdated server". In the launcher, make a profile for the server's exact version.

3. Modded server? Everyone needs the same mods

If the server runs a modpack, every player needs that same modpack, in the same version, installed through a launcher like CurseForge, Modrinth or Prism. A missing or mismatched mod usually shows up as "Mod rejections" or a list of mismatched mods.

4. Are they on the whitelist?

With the whitelist on, only names on the list can join, and everyone else sees "You are not whitelisted on this server". Add them with /whitelist add <name>, spelled exactly as their Minecraft name.

5. Are they typing the right address?

• If your server has a name like yourserver.example.com, they only need that, with no port.
• If it has an address with a port, like play.example.com:30123, they need the port too, after a colon.
• Bedrock players (phones, consoles, Windows) cannot use a Java server unless it has Geyser, and then they type the address and port in separate boxes.

6. Is the address brand new?

If someone tried your new address before it was set up, their computer can remember "this does not exist" for up to half an hour. Restart the Minecraft launcher, or on Windows run ipconfig /flushdns. If it works on a phone on mobile data, the address is fine and their computer just needs to catch up.

7. Banned, or on a cracked launcher?

Check /banlist. And a server with online mode on (the normal, safe setting) only lets in players who own Minecraft and sign in with a Microsoft account.

More guides

• How much RAM does a Minecraft server need?
• View distance vs simulation distance
• Letting Bedrock players join a Java server