For some reason the BattlEye Service in your game directory is outdated or not operating properly. All the latest BE files always come with the game distribution, so please make sure that it is properly installed and fully up-to-date (e.g. by verifying your game cache on Steam). Due to external factors in some cases the BE Service may have problems operating, which can also lead to this error. In that case please try to close all unnecessary programs while playing the game. If that still doesn’t help we would suggest a full repair reinstallation of Windows as a definite solution.
This answer is copied from BE's FAQ and slightly adapted to be Unturned-specific. Click here to visit the BE support page.