Sorry if you run into this error! It means the Steam client thinks you own the Gold upgrade, but when joining a game server the Steam backend thinks you don't. Relatively recently I investigated this bug but couldn't narrow down why it happens. I added this workaround launch option for the meantime:
- Right-click Unturned in your Steam Library
- Select Properties... to open the General tab
- Find the Launch Options field
- Type "-NoGoldUpgrade" without quotes