Hi cloa,
Thanks for using Apple Support Communities. It sounds like you’re having trouble making purchases. I’m happy to help.
To start, I suggest making sure that you’re viewing an accurate balance, and that what you’re trying to purchase doesn’t exceed that amount (unless you have another form of payment on file):
View your store credit balance - Apple Support
If you’re receiving an error message or another type of billing issue, I suggest reviewing the following:
Should the issue persist, you’ll want to contact iTunes Support for further assistance.
Take care.