I put an apple gift card on my account, but it still won’t let me buy an app app that cost money. What do I do?

I got the Apple gift card like four months ago and I’ve tried everything to try to buy an app using it but it won’t work. I bought a $10 Apple gift card and I was going to buy a game that cost. 699, but it kept giving me either a credit or debit card or a PayPal option and it didn’t have the Apple gift card option