I think there is an over the counter product called “thums”, it is meant to stop thumb sucking, but the idea is the same with nail biting. It tastes horrible and you would probably sooner eat your shoes than suck your thumb with this stuff on it.
I used regular clear nail polish which helped me stop my nail biting. My doctor at the time suggested duofilm (or its generic equivilants) which is made for warts. You dont want to chew on that stuff, but he claimed it was harmless to use on your nails. The nail polish worked so I didnt have to try the duofilm. Cant tell you if it works.