Hi, I'm Bhavik. Welcome to JustAnswer. I'm reviewing your question now, and will reply back ASAP.
Most of the time, a 503 error occurs because the server is too busy or because there's maintenance being performed on it.
You need not to worry about this error message. Nothing seems to be wrong with your mac book.It is just a temporary performance issue at Apple's end.
Please try using App store after some time. It should be able to work fine
Please let me know if this issue still persist ?