I tried the same tutorial I think (man on street), and this is the response I got from Boris support:
It sounds like something might be wrong with the background layer – It could be that it’s not tracked at a nearby frame, that not stacked behind the foreground layer (being removed), or maybe not marked for processing (with the cog icon). Can you quickly double-check all these things. If none of these help, can you relay me your AE project and, ideally, the clip being referenced (that you are removing from the cleanplate)?
I manage to remove what I can with a lot of messing around but no where as easy as that tutorial shows 😛