I bought n redeemed a $25 car and it won’t let me access the money to pay for iTunes subscription

There's nothing to access. Once you redeem the card, the money is added to your account and is automatically used for any payments that need to be completed. You don't need to do anything else. The subscription should be automatically charged from there before any other payment method on file.

If you have any issue with your payment method on file, then you will need to resolve that before it can use your account's gift card balance however.