Retaking your A level after graduating from a masters

Hi

I was wondering whether you are allowed to retake your A level after graduating from your masters as I had a bad experience with my A levels.

Yes, you are allowed. I can't imagine that many people would bother, given that any qualification tends to superced the previous ones.
Note that you'd likely need to study for the retakes independently, and sit the exams at a local exam centre as a "private candidate". (Each exam board has a "private candidates" page which will talk you through the process.)

Am I not allowed to go back to a sixth form