I live in Canada. Recently I paid my RBC CC off in full before the monthly due date. On my next bill I incurred interest called "purchase interest" Why?
The most likely reason that you were charged interest is that you did not pay off the entire balance the month before, carrying forward a balance. Depending on the terms of your credit card, you were charged interest on either the full balance or the remaining balance after your payment.
If that is the case, then you owe the interest. If that is not the case, or there is some reason why you think you should not have been charged interest, then call customer service and see if they will waive the interest (depending on the amount they may do a one-time waiver), or if they determine you were charged erroneously, ask them to reverse the interest change as well.