Unable to complete App Store region change on iPhone

Have you fulfilled all of the requirements in order to change your 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.

If so and you still cannot change your region see this

• If you can’t change your country or region, make sure that you canceled your subscriptions and spent your account balance. Complete these steps before you try to change your country or region.
• If you’re a part of a Family Sharing group, you might not be able to change your country or region. Learn how to leave a Family Sharing group.
• If you have less store credit than the cost of a single item, contact Apple Support.