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?