First UNCHECK solution.
The boxed version of Microsoft Flight Simulator for PC does include a download Key
https://helpdesk.aerosoft.com/hc/en-gb/articles/4574437978653-Does-the-boxed-version-of-Microsoft-Flight-Simulator-include-a-download-or-Steam-key-
Click “Start” > settings > update and security > troubleshoot > additional troubleshooters
Scroll down > click windows store apps > run the troubleshooter
Repair MsStore app.
Click “Start” > settings > apps > click microsoft store > advanced options.
Click > terminate > reset > repair
Restart PC
Also update OS (windows) via windows update.