Players get a version mismatch on your Terraria server

Players and the server must run the same version. Steam updates the players' game by itself, and your server updates itself at the next start while its version setting is latest. So after a game update, restart the server. If it still does not match, check that the version is not pinned and Auto-update server is on.

Fix it

1. Open Settings, Game Settings.
2. Check the version setting (Terraria version, TShock Version or tModLoader Version) says latest.
3. Click Show advanced (1) and check Auto-update server is on.
4. Restart the server and read the Console. A line starting with [pnh-update] says it updated, or that the newest build is already installed.

See How to update your Terraria server and hold a version.

On TShock

TShock follows a new Terraria within a few days. Until TShock releases its build for the new Terraria, players on the newest game cannot join. The server installs the new TShock at the first start after it comes out. Restart once a day until it does, or schedule a daily restart: How to schedule backups and restarts for your Terraria server.

On tModLoader

A server on an older tModLoader turns players away with You are not using the same version as this server. Restart to update. Players on Steam's tModLoader preview branch need a server on that preview release, or they switch back to the normal release in Steam. To put the server on a preview, use Edit Server Type on Overview, keep tModLoader and pick the preview at the Version step: How to update your Terraria server and hold a version.

When the player is on the older version

If the server updated and a player did not, the player updates the game in Steam, or restarts Steam to let it update.

Holding an old version on purpose

If you pinned a version, every player must use that same version too. Most players cannot pick an old Terraria version easily, so pin only when you have a reason.