The paint is not adhering to the wall board due to the sub strate finish and lack of primer. You should try to remove the paint, rough up the surface and prime before doing anything.

Try using a cheap alternative. Graffiti remover.

Since most vinyl wallboards have a glaze substrate, paint doesn't always adhere very well without a suitable primer designed for vinyl surfaces.

A biodegradable graffiti remover will often remove the paint from the surface, so that you can clean it, prime it, and repaint.

Ask your paint retailer for a vinyl-suitable primer AND the graffiti remover.