When I try to buy something it says to verify payment option and shows credit card and my gift card how do I use the gift card

I am not sure if this is your problem but see how to Add funds to your Apple ID https://support.apple.com/en-us/HT208687  Contact Apple support for help here: Get started." Try the "Get started" link for help with your problem.

And you can get Apple telephone support  for help in solving your problem using this link https://getsupport.apple.com/?caller=kbase&PGF=PGF63005&category_id=SC0245&symptom_id=23362 Be patient sometimes Apple support is closed to immediate phone support.

If you have funds on your account, normally the cost will be deducted from the account funds before the credit card is charged. You shouldn't have to select the Gift Card funds to make them primary.