First of all, please ensure that BE is properly installed for your game. You can manually install it here:
-
Right-click Unturned in your Steam Library
- Hover the Manage button
- Click Browse local files in the dropdown
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Open the BattlEye folder
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Run Install_BattlEye.bat
If the issue still persists your system’s root certificates might be broken/corrupted. To fix them download and install the following update from Microsoft: http://support.microsoft.com/kb/2813430.
This answer is copied from BE's FAQ and slightly adapted to be Unturned-specific. Click here to visit the BE support page.