Computer problems help!?

I'm trying to create a hulu account with my $100 shop etc. Gift card which is the same as an American express giftcard. I enter all the numbers but it wont let me enter the last number so I can't sign up ;( what should I do? I'm only 13 so I don't hve a credit card and my mom Wont let me use hers is there a way to fix this?