Outdated client or outdated server

It says "Outdated client" or "Outdated server".

Your game version and the server version differ: switch the game to the server's version, or add ViaVersion to let other versions join.

Before you start: Know the server version (shown in Versions and the console at startup).

Step by step

1. Check the server version in the panel's Versions tab.
2. In the Minecraft Launcher: Installations > New installation > choose that exact version > Play.
3. Or, to let players on newer clients join a Paper/Purpur server, install ViaVersion from the Plugins tab (add ViaBackwards for older clients).
4. Restart the server after installing the plugins.
5. Modded servers (Fabric/Forge) need the exact same version and mods on every client; ViaVersion does not help there.
6. Tell players the version in your MOTD (Quick Settings) so they pick the right one before joining.

Check it worked: Players on the intended versions can join.

Why it happens: Minecraft only connects matching protocol versions.

More guides on Joining and connection problems:

• Find your server address and connect
• Connection refused
• Connection timed out or server shows offline in the list
• Failed to verify username or invalid session
• Not whitelisted or banned from your own server
• Friends cannot join but you can