Unable to change App Store region after downloading an app

Have you gone thru all these steps?

Copied from: Change your Apple Account country or region - Apple Support

What to do before you change your country or region

• Spend any balance remaining on your Apple Account. If you’re not sure whether or not you have a balance, check your Apple Account balance.
• Cancel any subscriptions that block a country or region change, and wait until the end of the subscription period. 
• Wait for any memberships, pre-orders, movie rentals, or Season Passes to complete, and wait for any pending store credit refunds to process. You can cancel a pre-order.
• Make sure that you have a payment method for your new country or region. See what payment methods you can use.
• Redownload your apps, music, movies, TV shows, and books on any devices that you might use to play or read them in the future. Some types of content might not be available in your new country or region. Learn what types of content and services are available in your new country or region.
• After you update your Apple Account billing address and payment method, you can upgrade your iCloud+ plan using the pricing in your new country or region.

Some users have encountered an issue of only being able to change the region every 90 days.

M1 Mac mini, macOS 15.3;  Watch, series 10 GPS-only, watchOS 11.3; iPhone 16, iOS 18.3; TV 4th Gen, tvOS 18.3; HomePods, audioOS 18.3 and Airport Express w/ Airplay 2

What happens when you click on "Learn More?"

You will have to wait and see what happens. Fraudulently changing your region to download apps or use services not available in your physical region is a violation of the Terms of Use. Some users have found their Apple ID locked. If that happens, you may need to create a new Apple ID, or factory reset your device which will cause it to wipe all data on it.