Hi…,
Welcome.
Please see 16. Export Flight Plans from Navigraph Charts to Microsoft Flight Simulator 2020
When you first export your .pln from Charts , navigate to and choose your MSFS LocalState directory where there should be other .pln plans.
Depending on how you obtained MSFS, this will likely be something like
Sim from Marketplace:
%localAppData%\Packages\Microsoft.FlightSimulator_8wekyb3d8bbwe\LocalState
Sim from Steam:
%appData%\Microsoft Flight Simulator
This directory should now be remembered for subsequent Charts exports.
Cheers
Ian