This is due to an issue with the MS Store / XBox app not “seeing” your PC.
Start by resetting the Windows store:
Close the Microsoft Store app if currently open.
Press the Win + R keys to open the Run dialog.
Type WSReset.exe into Run, and click/tap on OK.
A command prompt will now open without any message. After about 30 seconds, the command prompt will automatically close, and the Microsoft Store app will open.
After that try to log in again.
If that does not work:
Update Windows
Ultimately, you may have to reset Windows since this issue arose from a hardware change:
https://support.microsoft.com/en-us/windows/recovery-options-in-windows-31ce2444-7de3-818c-d626-e3b5a3024da5
If you have any more issues, feel free to submit a support request with us:
Submit a request – Microsoft Flight Simulator Support (zendesk.com)