After a lot of hours spent resetting and uninstalling MS Store App, Xbox App and/or MFS … these last steps worked for me. MFS is installing fine 
Make sure windows is updated and the ms store app is updated.
Open the Windows Security, Virus and Threat protection settings … turn off Real time protection, etc 
Open the Microsoft Store and signoff your profile.
Open the Xbox App and signoff your profile.
Start/Click MFS and login your profile when requested…let the fun begin 
That’s all folks
Hi camechi
Thanks for sharing. Might point out though, you would have found 4 threads on this issue if you had used the search function before opening your thread. Turning off the real time protection is also one of the solution mentioned there. But no harm done.
Happy flying.
Thanks @VeryOldPilot.I did check two or three of the thread. The steps listed on them did not get my sim up and running, though.
hope to see you in the sky 
Happy to read that you found the correct solution for your own issue. Perhaps your posted method might help someone else - in this guessing game some users are left in.
Avoids having to to rename MS Flight Simulator 2020 to MS Virus Simulator VS2020 (joke)
Happy flying
Now you can play with real-time protection - on.
In windows seacrh bar type
powershell
right click on powershell and select run as administrator
Copy/paste to powershell window
Add-MpPreference -ExclusionProcess “C:\Program Files\WindowsApps\Microsoft.FlightSimulator_1.10.8.0_x64__8wekyb3d8bbwe\flightsimulator.exe”
(right click to paste into powershell window)
and press enter
Run game.
source https://forums.flightsimulator.com/t/simulator-doesnt-start-anymore-at-all/314155/247?u=tenpatrol
I re-enabled my Virus and threat protection settings after awhile… MFS is still behaving as desired. I also added some exclusions … (folder) c:\MicrosoftFlightSimulator … (process) FlightSimulator.exe.
Thanks @TenPatrol for the powershell command.
If we upvote this enough Microsoft Studio may just give Sec/Admins something nice to play with 
Come to think of it @VeryOldPilot how do we get paid for consultancy if this works 
