Hello
When I try to fill the bottom face, this is what I get:
The top side is filled and doesn't have this issue. The problem seems to come from the bevels but I don't get why this happens. Any idea?
Hm, this has usually something to do with an incorrect selection, although I can't see what you've done here from this screenshot alone...there might also be some 'double Vertices' involved...
Remove that horror Face, Select everything and M > Merge > By Distance, than try filling the Face again...
If that doesn't help, could you upload your .blend file to Dropbox, Googledrive, or any other Filesharing sevice and post a link here?
For some reason trying to merge on everything didn't work out but merging only on the problematic vertices did work! Thank you so much Martin!