Apple Account in credit but can't spend it
I recently received an Apple Gift Card as a birthday present. I have redeemed it into my account which is therefore now in credit. I want to use that credit to make purchases on Apple TV, however when I try to make a purchase it wants to charge my debit card.
I have taken this up with Apple Support via emails but after several days of emails they are still unable to sort out the problem for me and seem to have stopped responding.
The stage at which we left things was that apparently my primary payment method is set up as PayPal. I believe there may also be an issue in that I use Family Sharing with my wife, for the sole purpose that we can each have our own Apple Music libraries via the one account. The monthly Apple Music payment is invoiced to my account via PayPal.
I do not understand why the above scenario means I cannot spend my Account Balance on Apple TV. Can anyone explain?
iPhone XS, iOS 15