I’m not paying for a app that is free to start with

Unfortunately you do not provide the exact error message which tells us from the start what the issue is.

If a message says 'billing problem with previous purchase' or 'verification required', try the things in this article -->

If a message says 'billing problem with previous purchase' or 'verification required' - Apple Support

You get a billing problem message because there is a problem paying for something a person bought in the past and this needs to be resolved before Apple will let you make a new purchase, even free items. It isn't a problem with a specific item, it's with the payment method used by you or—in the case of Family Sharing—the Family Organizer.

Get the payment method owner to review their payment methods as in this document: 'Add a payment method to your Apple ID' on an Apple mobile device, a Mac, or a PC; click here --> Add a payment method to your Apple Account - Apple Support

- Payment methods with a problem will appear in red. This document tells you how to update problem payment methods --> If you need to change or update your Apple Account payment method - Apple Support

- If the person is updating an old card and has ongoing subscriptions the person may need to first add the new card and make it the primary payment method before deleting the old card.

Related documents:

If the payment method is declined in the App Store or iTunes Store, click here --> If your payment method is declined in the App Store or iTunes Store - Apple Support

"There is a billing problem with a previous purchase." = "you have an unpaid balance in the App Store or iTunes Store"

- 'If you can't remove your last payment method or use no payment method with your Apple ID' click here --> Remove a payment method from your Apple Account - Apple Support

If none of the self-help solutions resolve your situation then ask Apple for help. Click here --> Choose your country or region - Official Apple Support Select your country, then a product. If you don't see one that handles your issue then keep experimenting with selections until you reach one that gets you a chat session or a telephone call and get the representative to redirect you.

or:

Contact Apple for support and service --> "See a list of Apple phone numbers around the world." - Contact Apple for support and service - Apple Support

Note that Apple may require the presence of a credit or debit card even if an item is free. They won't charge you but the card has to be there.