Cannot report a problem

Hello,

I was charged for a purchase via app store. Since I do not remember I agreed to pay for these services, I would like to cancel this purchase. However, when I try to report a problem and cancel a puchase, I cannot sign to my apple ID, since when I type my apple ID and password, the same page appears again (without any notification about wrong password or wrong name). Please help me to sign in in order to report a problem.