"Built-in resources mismatch."

This means the non-moddable assets loaded by your client don't match the ones the server knows about. 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

If the error occurs even after those steps, it may be worth deleting the built-in resources, forcing them to be re-installed:

1. Right-click Unturned in your Steam Library
2. Hover the Manage button
3. Click Browse local files in the dropdown
4. Delete the Unturned_Data folder
5. Repeat the Verify integrity of game files steps