What it means
Minecraft only lets a game join a server on the same version. When they differ, the player sees one of:
- Outdated client! Please use 1.21.4: the player's game is older than the server.
- Outdated server! I'm still on 1.20.1: the player's game is newer than the server.
- Incompatible client! Please use 1.21.4: the same thing, worded differently.
Fix it for one player
Change the version in the Minecraft Launcher:
- Open the Installations tab.
- Click New installation.
- Pick the server's version from the Version list and save.
- Play using that installation.
Change the server's version
If most of your players are on a newer version, update the server instead. On the Startup tab, pick the version and restart. Take a backup first: a world opened in a newer version can't go back to an older one. Plugins and mods also need versions that match.
Let everyone join, whatever their version
On Paper, Purpur or Spigot, install these from the Plugins tab:
- ViaVersion lets newer games join an older server.
- ViaBackwards lets older games join a newer server (it needs ViaVersion too).
Players then see blocks and items from their own version where possible. It's the usual answer for public servers where people use all sorts of versions.
Modded servers
On Fabric, Forge and NeoForge, players need the exact same Minecraft version and the same mods, with matching versions. ViaVersion doesn't help here. The easiest way is to have everyone use the same modpack.
Questions
Which version is the server on?
It's on the Startup tab, and in the console when the server starts.
Can a 1.21 player join a 1.20 server?
Not on their own. Install ViaVersion on the server and they can.