Hi there, wasted a full bank holiday day off trying to suss out this problem and got it eventually, well not a full day, but I was very frustrated at the end of it all. So here it goes.....Sign In to iTunes Store, click 'account', then click 'edit' for 'Billing Information'. You already have your credit card details entered, so click 'none' in choice of 'Payment Method' and all your Payment Card details will be erased, click 'done' on the bottom right hand corner of window. Now you will return to your Account Information page and 'no credit card on file' can be seen where originally you could see the last 4 digits of your credit card. Click on 'home' or 'films' and continue with a purchase using the 'iTunes Gift Card' credit on your account. 😎