Unable to purchase my balance

Your question is not very clear.

Your balance is the amount you have on your account you can use to purchase things.

Balance can also be used to refer to what you owe on your Apple Card if you have one and have used it to purchase things in stores

You don't purchase a balance.

Can you explain exactly what is happening, and if you get any error, what it says?

Are you trying to pay what you owe your Apple Card?

Are you trying to us funds on your Apple ID to buy something in one of the stores?

Are you trying to do something entirely different to either of these?