I can’t subscribe with my Apple ID balance

How apps, content, and subscriptions from Apple are billed - Apple Support

Some purchases aren’t charged to your Apple Account balance

- If you're in a Family Sharing group, purchases that you make are charged to your personal Apple Account balance. If you don't have enough Apple Account balance to pay for the purchase, the remainder is charged to the family organizer if purchase sharing is turned on.

- You can’t use your Apple Account balance to buy gifts or gift cards.

- Some subscriptions might not be charged to your Apple Account balance (and will require you have a payment method on file).)

As I wrote previously, in the case of subscriptions Apple may require you to have another payment method present on your account even if eventually the money will be taken from your personal balance. In general, unless you are getting something that is absolutely free, you will have to have a payment method such as a credit or debit card.