BattlEye "Bad Client Version"

For some reason the BattlEye Client in your game directory is outdated. All the latest BE files always come with the game distribution, so you will just have to make sure that it is properly installed and fully up-to-date (e.g. by verifying your game cache on Steam).

This answer is copied from BE's FAQ and slightly adapted to be Unturned-specific. Click here to visit the BE support page.