You aren't getting any eggs because they are in different egg groups. Cinderace is in the Field and Human-Like egg group while Gengar is in the Amorphous egg group. If you want Sucker Punch, I would recommend you breed a female Cinderace with a Male Galarian Mr. Mime that knows Sucker Punch. You could also breed Cinderace with Inteleon, Shiftry if you have Sword or Theivul if you need to. Keep in mind they need to be make though.
Hope I helped!