Why can't i use my account balance to pay past due?

because you can't use account balance to pay a past due amount. https://support.xbox.com/billing/xbox-subscriptions/pay-past-due-balance-on-xbox-subscription

Unfortunately, you can't use your account balance to pay for a past due amount.

You will have to
go to this page here to pay your past due amount of money, and use your bank account or credit card with sufficient funds.

After you payed your past due amount, you can then redeem your 3 month card again. You can't redeem any prepaid codes with a past due amount on your account.

Hi Ruben,

Thanks for reaching out to us. We understand having issue paying for subscription. If the subscription is past due, you are unable to use account balance to pay for it. IF it isn't past due, there may not be enough in the account balance to cover the tax.
That's why it asking for card.

Thanks,

Brad

you can't pay a past due with your account balance.

If you ended up in a past-due situation, that is the reason why it will not resume by just adding credit to your balance. Unfortunately gift cards, subscription cards, or your Microsoft account balance can't be used to pay
for a past-due Subscription so, to settle that, you will need to use another payment method or eventually contact support to see if they can manage that.

https://support.microsoft.com/en-us...unt-pay-for-a-past-due-microsoft-subscription

https://beta.support.xbox.com/contact-us