Billing address.

What does it say when it doesn't let you? Unfortunately you have provided insufficient information for us to propose a specific solution. Describing what happens when you try, and the exact wording of any error messages, would greatly aid us in helping you. In the meantime review these documents if you think they apply:

Click on this link "If you can’t download or update apps on your iPhone or iPad " --> https://support.apple.com/HT207165

If the first 5 don't help, use the 6th.

Considerations:

Do you have the same problem when using a different Wi-Fi network?

Does restarting your device make any change?

Can you download apps normally on another device?

If a feature on your device is not working, it may have been disabled. See how to 'Use Screen Time on an iPhone, iPad, or iPod touch' by clicking here --> https://support.apple.com/HT208982 Go to: Content & Privacy Restrictions > iTunes & App Store Purchases

If instead you see a message that says "Your account has been disabled in the App Store and iTunes" click on this --> https://support.apple.com/HT208856

Resolve issues between iTunes and third-party security software - https://support.apple.com/HT201413 - You can’t connect to iTunes Store to buy or download content.

If you see 'app needs to be updated' after updating to iOS 14.5, click here --> https://support.apple.com/HT212310

Have you entered a payment method? This document explains how to 'Change, add, [review,] or remove Apple ID payment methods' on an Apple mobile device, a Mac, or a PC; click here --> https://support.apple.com/HT201266

Apple Recommended article by ASC contributor Urquhart1244 --> https://discussions.apple.com/thread/8434425 with various suggestions about network, payment and log-in issues.

Check Apple's system status --> https://www.apple.com/support/systemstatus

"If you can't connect to the App Store, iTunes Store, or Apple Books" click here --> https://support.apple.com/HT201400

- If none of the suggestions work, try the last one: "If you’ve tried all of the above steps and you still can’t connect, contact Apple Support."

Log into your account in the App Store/iTunes and you should be able to change/verify it there. Or you can click the blue Edit payment methods in the article below. When signed in and the page loads, you will be on the credit card page. After updating that, click the Go Back button at the lower left and you will be on the address page.

This is the same link Limnos posted above. Apple ID payment information - change.  

See the Apple Support article(s) below.

Apple ID without a credit card  -

Apple ID - If you can‘t choose None when you edit your Apple ID payment information.   *

That's the eighth paragraph I posted. :-) it wants a payment method.