Why is my app not letting me purchase something within the app? I have money on my Apple account but it keeps asking for another payment method

The resolution to this issue is often making sure you have entered a credit card or other valid method of payment* even if you don't plan on needing to use it.

If there is an option to select "Review" or "Continue", select that and enter valid payment information if required (you may need to enter your Apple ID password first). If there's problems doing this:

Tap: Settings > [your name at the top] > Media & Purchases > View Account > Review Apple ID.

More things to try in this 2018 post by Urquhart1244 marked Apple Recommended:

https://discussions.apple.com/thread/8434425?answerId=33542414022#33542414022

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 - https://support.apple.com/HT202631