I've noticed previously that the mobi files have the "prc" extension, but since I'm not a Kindle user I've never had to figure out what that means logistically. This thread seems to indicate that you have to have your Kindle physically connected to your computer, then drag the file over: https://www.amazon.com/forum/kindle?_encoding=UTF8&cdForum=Fx1D7SY3BVSESG&cdThread=Tx18FQYYZ3V6WFX

A couple of other recent discussions suggest that Kindle used to directly import prc files but something changed a year ago, and now the cable+drag&drop method or converting to mobi via Calibre or the like is necessary. If this is the case, then Bella may need to update their instructions. (But complaints/suggestions always come more strongly from customers.)