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 - 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 ID' 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 ID - Apple Support but note that Apple Account balance may not be sufficient in this situation.
Note that any apps you downloaded with the old account will now have to be deleted and downloaded with the new account, or you have to set up Family Sharing between the two accounts. To learn how to set up Family Sharing, read this support article --> Set up Family Sharing - Apple Support