Couldn't change the Country and Region on my iPhone
There could be numerous reasons for that error. So, a good starting point is to make sure you check all the below mentioned points-


Ensure that there is no Store credit remaining in your Apple ID.
Check whether any refund is in progress associated to your Apple ID.
Turn OFF all the subscriptions that you have for various services with your Apple ID.
Make sure you are not in a Family Sharing group.


Once you are assured with all the above points, you can try changing your Country or Region now. In addition, make sure to enter the correct details, especially Payment information, Address, and Phone.