How can I purchase iCloud storage space using Apple Account credit?

First, make sure you understand the difference between iCloud Storage and storage on your iPhone. It's important to know that buying more space in the cloud will NOT increase the amount of physical storage on your iPhone --> What's the difference between device storage and iCloud storage? - Apple Support

Assuming you need the space in iCloud, you say you can't because you have some credit. You have some credit, what exactly? Do you not have a valid payment method on your account? This link about adding a payment method you can use, might help you --> Payment methods that you can use with your Apple Account - Apple Support