How do I use my credits? What are the steps?

See -> Add a payment method to your Apple ID

Add a payment method to your Apple ID – Apple Support (UK)

• 👉 if you use your Apple Account balance, you need to set your Apple Account balance as your default payment.

but when I press done it just loads

Check first the network connection:

You need to have a good Service and/or Wi-Fi.

If they are On, turn Off Hotspot & VPN.

Charge the battery > 70%.

If you can't connect to the App Store, iTunes Store, or other Apple services

If you can't connect to the App Store, iTunes Store, or other Apple services - Apple Support

👉 If you tried all of the above steps and you still can’t connect, contact Apple Support.

Get support from Apple directly -> Get Support or call Apple in your region see -> Contact Apple for support and service or check -> here for more support and assistance.