I'm having issues installing version (X) of Unreal Engine

If you're experiencing issues installing a particular version of Unreal Engine it may be because the NDI Unreal Engine SDK requires that Unreal be installed prior to installation. The installer checks for registry keys in this install location

HKEY_LOCAL_MACHINE\SOFTWARE\EpicGames\Unreal Engine<version>\InstalledDirectory.

If the registry keys are not there, then the installer will be unavailable and appear grayed out for that particular version.