How to do I use the funds available on my Apple ID balance? It only asks me to redeem it which I need to enter a gift card?

Before you can add funds to your Apple ID, you must have a valid payment method on file. Learn about the payment methods that you can use with your Apple ID. ... See how to Add funds to your Apple ID https://support.apple.com/en-us/HT208687  Note that "after you authorize the payment, the funds ... [are credited] to your Apple ID. Then, you can use the credit to make purchases from the App Store or iTunes store. Need more help? Contact Apple support for help here: Get started

Do you have a credit card on file? What happens when you try to make a purchase? If you are getting an error message, what is the exact error message? If you are trying to make a purchase, is your account balance enough to cover the cost plus applicable taxes?