A Microsoft file hosting and synchronization service.
Hello
You are not talking to support here. This is community forum, so no one can actually access your account to review it for you.
However, can you share the exact error message if there was any? What did it say when you tried to purchase the additional 1TB storage?
Also, was your existing Microsoft 365 personal signed up in a different country other than Vietnam?
If you go to your profile https://account.live.com > Your info, is the country Vietnam?