This means the game code running on your client doesn't match the code running on the server. A good first troubleshooting step is to verify game files which repairs any corrupted or missing files, and checks for updates:
- Right-click Unturned in your Steam Library
- Click Properties...
- Click Installed Files in the left column
- Click Verify integrity of game files
Unless you manually installed something into the Unturned directory the most likely cause is that a file got corrupted. Either way, verifying game files should replace it. Ordinarily the game's code should only be modified by an official update to the game, but in that case a different error message will be shown saying the version number is different.