"This server is using modified code."

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:

1. Right-click Unturned in your Steam Library
2. Click Properties...
3. Click Installed Files in the left column
4. 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.