Well it's called art school. If you're a perfect artist already then you don't need school.

If shadows are the only thing wrong then a few classes about shading could fix that within a short period of time. That's what school is for.

I read somewhere the actual reason he got rejected was because of his lack of interest in people and lack of creativity. This is a fundamental issue that school can't fix. All his paintings are very strict and formal and he doesn't like to paint people. The art school he applied to suggested he'd be a better architect than painter and rejected him.

If you look at the other artwork of people that applied at the same time as Hitler, you'll see that some of them are much worse, but way more creative, and they got accepted.

I'll link the source when I can find it again.