Straight female here. I’m really interested in being submissive in bed but I can only cum when I’m on top during sex. Aren’t subs not allowed to be on top?

Specific Doms may vary but a sub can do anything their Dom orders them to. If serving your Dom means riding his cock on top, then that’s submission. If your Dom wants you to peg him, that’s submission. Of course, your Dom may not be overly concerned with your orgasm, but giving him this information in advance equips him to give you a treat if you’ve earned it.