Greetings Chloe_alxndr,
Thanks for visiting the Apple Support Communities! I understand Apple ID balance is the only payment method associated with your Apple ID, and you're unable to make an in-app purchase.
Have you made a purchase in the App Store previously? If not, check out the following information: "In some cases, like if you're using an existing Apple ID with the App Store for the first time, you must provide payment information." - If you can't remove your last payment method or use no payment method with your Apple ID - Apple Support
When adding a credit card to your Apple ID payment methods, your Apple ID balance will be charged first for your in-app purchase: How App Store and iTunes Store purchases are billed - Apple Support
Cheers.