How do I activate my Apple Account to download apps?

How to fix the "Apple ID not yet used in … - Apple Community

This post at that link implies you can do this without actually adding a payment method.

1. Go into https://music.apple.com (if you can't do it on the app, try doing it in your computer).

2. Sign in and accept Terms and Conditions.

3. Go back to App Store and install whatever you want.

If the above does not work, the resolution to this issue is often making sure you have entered a credit card or other valid method of payment* even if you plan on using account balance to pay for a purchase or the item is free; e.g.: Sign in to your Apple Account on the web or in your device settings - Apple Support

- "When you sign in to the App Store and other media services for the first time, you might need to add a credit card or other payment information."

If there is an option to select "Review" or "Continue", select that and enter valid payment information if required. If there's problems doing this:

On a mobile device tap: Settings > [your name at the top] > Media & Purchases > View Account > Manage Payments > Add Payment Method.

Alternatively this document explains how to add a payment method to your Apple Account on an Apple mobile device, a Mac, or a PC; click here --> Add a payment method to your Apple Account - Apple Support

If you continue to have problems contact Official Apple Support - Select your region from the list and then your country from the pop-up window. If you do not see your country, see if there is an "other" listing for that region.

https://getsupport.apple.com/?caller=cups&PGF=PGF63005

*Payment methods that you can use with your Apple ID - Payment methods that you can use with your Apple Account - Apple Support but note that Apple Account balance may not be sufficient in this situation.