This happens because you permanently declined BattlEye installation by clicking Do not ask me again in the BattlEye Launcher window previously.
To fix this, go to the BattlEye folder in your game directory (e.g. “Fortnite\Binaries\Win64\BattlEye” in Fortnite) and double-click the Install_BattlEye.bat file to install BattlEye.
For more information, see BattlEye's official FAQ here.