Amazon wont let me use my gift card balance?

hard one to answer without seeing what you are trying to do, but a few things:

1. Does what you are buying + SHIPPING + TAX come to under the $25 ?
2. you can not buy gift cards with a gift card balance, does your order contain one?
3. have you some how un-selected the gift card as your form of payment?

past that I would give amazon a call.

some places accept debit gift cards, not all gift cards work everywhere. Look on the paperwork that came with the debit gift card and you'll see if they accept it or not. So they're asking you to pay with some other card (or you don't get the product).

they obviously are not accepting your gift card