What can't I use my Apple balance to pay for in app subscriptions?

For most subscriptions, yes. Apple doesn't provide a list.

How App Store and iTunes Store purchases are billed. Click here --> How apps, content, and subscriptions from Apple are billed - Apple Support

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

To learn 'What you can buy with your Apple ID [Apple Account] balance', click here --> https://support.apple.com/HT211187

If you want help with the payment problem you need to tell us why you think it isn't working, including exact error messages.

Apple says: "Some purchases might require that you have a credit card on file, even if you use your Apple ID balance to make the purchase." - from: What you can buy with your Apple ID balance --> What you can buy with your Apple Gift Card or Apple Account balance - Apple Support

For a variety of reasons Apple will almost always require a functional payment method such as a credit or debit card be present when downloading anything from the iTunes or App Stores that isn't totally free (so, for example, a "free trial" that turns into a paid subscription isn't ' free'), even if the item will be paid for with Apple Account balance. The only solution to satisfy this requirement is to add a valid 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 --> Add a payment method to your Apple ID - Apple Support

what cant i use my apple balance to pay for in app subscriptions

See -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

• if you want to use your Apple Account balance, you need to set your Apple Account balance as your default payment.