The "unable to process the request" loop is a documented server-side issue that usually occurs when a billing profile is internally flagged, corrupted, or missing a critical regional configuration. Since you have already ruled out browser, device, and network issues, you can attempt one final self-service fix before escalating to Microsoft's specialized billing agents.
I suggest contacting the Microsoft Live Billing Support
- Open the Microsoft Contact Support Portal.
- Sign in with the exact Microsoft account you are trying to configure.
- In the search bar, type Billing and payment updates and press Enter.
- Scroll to the bottom and click the blue Contact Support button.
- Under Products and Services, select Microsoft 365 and Office.
- Under Category, select Billing and Payments update.
- Click Confirm, then select Chat to initiate a secure, live session.
Hope this helps