I painted the interior of a melamine cabinet (after sanding and prepping it) with all-in-one paint and primer.
The paint now scratches off very easily. Is there a sealer I can put on it to avoid this instead of repainting?
You could try a clear wood sealer but the problem will still persist. The true fix is to strip off the current paint, then properly prime and repaint with a good cabinet paint like Benjamin Moore Advance.
As mentioned, you should STRIP off the old poorly chosen brand of paint first, then redo it with first, an Oil Base sealer-primer, then finish coat with a GOOD Brand Acrylic Paint (semi gloss) from Sherwin Williams… Good Luck