According to the rules, ANY engine you can imagine is allowed. For what I understood, "game engine" includes every piece of software that could be used to make the base of a game work. I haven't used Scratch that much but from what I can tell, it probably could be used to make a game work. However, consider that while it might be easier to use for beginners, on long term it might have some limitations that could be easily solved with a more complex programm. (I don't know how to code so I use Construct 2, and I'm aware that maybe if I knew how to use Unity, I could make better games).
So in my opinion, you should be allowed to use Scratch, since it could be considered as a game engine. However, this is not an official response, just my thoughts on that. While you can stay with my opinion, I would still recommend you to ask a mod directly, so you can get an official response, but as I stated, from my point of view, there's no reason why your submission would be removed just because you used Scratch.