Cannot make a purchase with my $7 balance

It helps to include any error message because that indicates what exactly may be the problem. Do you see a message about needing to add a payment method? If yes, Apple requiring a payment method such as a credit or debit card, even for free apps or where you have enough account balance to pay for the item, is not an uncommon observation. Some possible reasons are outlined in the links below such as outstanding balance, an issue with a payment method you have currently entered, ongoing subscriptions, or simply because they say so. You can try to configure your account to not have a payment method but you may find there is no option other than to enter a valid payment method to satisfy one of Apple's requirements. If the item is free you will not be charged. If it costs something, your Apple ID balance will in most cases be used first. You can later try to remove the payment method and see if it only needs to be there temporarily.

If you can’t download or update apps on your iPhone or iPad - https://support.apple.com/HT207165

- "You might need to have a payment method on file, even if the app that you want to download is free." If you have something else on your account that requires a payment method, such as a subscription, and your current payment method is not working then Apple won't let you download even free things until you fix the payment method. This is explained in this document, 'If your payment method is declined in the App Store or iTunes Store' - https://support.apple.com/HT203005

If you can't remove your last payment method or use no payment method with your Apple ID - https://support.apple.com/HT203905

- If you have subscriptions like iCloud+, you must keep at least one payment method on file.

- You might not be able to remove a payment method if you have a balance due.

https://support.apple.com/HT204053 - at the very bottom: "When you sign in to the App Store and other media services for the first time, you might need to add a credit card or other payment information."

Also:

How App Store and iTunes Store purchases are billed - https://support.apple.com/HT201359

Some purchases aren’t charged to your Apple ID balance

- You can’t charge gifts to your Apple ID balance. If you send a gift or gift card, your primary payment method is charged.

- Some subscriptions might not be charged to your Apple ID balance.

And:

https://support.apple.com/HT211187

"Some purchases might require that you have a credit card on file, even if you use your Apple Account balance to make the purchase."

So, the resolution will be to add a payment method:

This document explains how to change, add, review, or remove Apple ID payment methods on a Mac or Apple mobile devices; click here --> https://support.apple.com/HT201266

If you still have problems Click here --> https://support.apple.com/choose-country-region 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." - https://support.apple.com/HT201232

Could be for a number of reasons. It may help to know if you are seeing a message or what happens when your try:

• Are you using it for an Apple purchase or trying to use it for a purchase some place else? The Apple Account balance is only for purchases managed by Apple.
• Tax is added to the purchase price and varies by region, for example if the purchase is $6.99, the amount you have in your Apple Account may not cover the charge including tax.
• You may have a previous purchase that has not been paid. You would need to pay for the purchase before you are allowed to make any other charges. You can use the steps under See your purchase history on your device in this Support Article for viewing your Purchase History. Any amount in Red or showing as Pending has not been paid yet. You may need to adjust the filters to view all transactions as the example in the Support Article is showing Last 90 Days and Paid transactions.

https://support.apple.com/en-us/118212?device-type=iphone

• Your account may be disable for Purchases and need to be reactivated.

https://support.apple.com/en-us/108100

• You may have Screen Time restrictions place on your account to prevent in-app purchases.

https://support.apple.com/en-gb/102470

• You may still need to have a valid card on file to make purchases, even when you have funds in your Apple Account. As seen in the Support Article here, it says: You might need to have a valid payment method on file, even if the app that you want to download is free.

https://support.apple.com/en-us/102632