/ is a special character, escape it
[01-20]\/[A-Z][01-10]
This information is also available from a site as Rom said.
Ah, Raimo saw Chris’ answer did answer the ‘/’ but was not the complete solution. Kuddos for getting the full answer, but I can never pass a challenge like this to see if the regex can be shorter. And it can:
(([0-1]{1}[0-9]{1})|20)\/[A-Z](?1)