How to add funds to my apple id?

See Add funds to your Apple ID https://support.apple.com/en-us/HT208687

"Before you can add funds to your Apple ID, you must have a valid payment method on file. Learn about the payment methods that you can use with your Apple ID. ... See how to Add funds to your Apple ID https://support.apple.com/en-us/HT208687  Note that "after you authorize the payment, the funds ... [are credited] to your Apple ID. Then, you can use the credit to make purchases from the App Store or iTunes store. Need more help? Contact Apple support for help here: Get started."

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I can't guarantee a fix, but also see:

Why is my Apple ID setting greyed out on iPad? https://discussions.apple.com/thread/8355468

Apple ID banner grayed out https://discussions.apple.com/thread/8611970

My Apple ID is Greyed Out in Settings, Fix https://macreports.com/my-apple-id-is-greyed-out-in-settings-fix/

If necessary, contact iTunes Support. See the Apple Support articles below for contact information.   

iTunes Support – world wide 

Apple Support schedule phone call       

Clarification question - did you update you address?

Apple ID - Add funds to your ID