Cannot open an .mpp file
You double-click an .mpp file but nothing happens or Windows asks which app should open it.
Likely fix
Confirm you have Project Plan 3 or 5 assigned and that Project Professional is installed from your Office 365 portal — Project Professional is not part of Microsoft 365 Apps.
What to collect
Send the .mpp file path, your Project license if you know it, and whether Project Professional appears in your Start menu.