https://forum.pmdg.com/forum/main-forum/pmdg-737-for-msfs/229281-importing-flight-plan-from-simbrief-while-on-ground
You are importing to the incorrect location, do NOT use the path with "Community" in it, that one only gets read when the sim starts, the locations below are dynamic:
737 FP
C:\Users\YOURUSERNAME\AppData\Local\Packages\Micro soft.FlightSimulator_8wekyb3d8bbwe\LocalState\pack ages\pmdg-aircraft-737\work\Flightplans
738 FP
C:\Users\USERNAME\AppData\Local\Packages\Microsoft .FlightSimulator_8wekyb3d8bbwe\LocalState\packages \pmdg-aircraft-738\work\Flightplans
736 FP
C:\Users\vstar\AppData\Local\Packages\Microsoft.Fl ightSimulator_8wekyb3d8bbwe\LocalState\packages\pm dg-aircraft-736\work\Flightplans
For Sim from Steam these would be
%appData%\Microsoft Flight Simulator\Packages\pmdg-aircraft-737\work\Flightplans
%appData%\Microsoft Flight Simulator\Packages\pmdg-aircraft-737\work\WX