Can I manually add cover art to my files as engine has not added them automatically?
You can use any tag editor to add album art. I use MP3tag, it’s free and works well.
I think, he meant adding cover art by engine prime. That is not possible at the moment i guess. I use mp3tag aswell. But it would be a great feature in engine prime in future or maybe to set in all Tracks without cover art your own design automatically