What is wrong with a cat who has a scab on his nose?

See a vet and make sure he doesn't have mites, a nose infection/problem, or Allergies, etc. There might be an irritant on his nose and he's scratching it with his claws. Alternatively, he might have gotten into a fight with a cat or another dog or another animal, so he'll need to be checked for rabies, etc. The vet will check him and make sure there isn't a serious problem.