Ran into something similiar that a Steam purchased MSFS2020 you cannot bujy anything from the marketplace.
Ensure you have the Steam Overlay enabled in GEFORCE EXPERIENCE, then in STEAM go into the MSFS2020 properties and enable the Steam Overlay as well.
https://flightsimulator.zendesk.com/hc/en-us/articles/360016063399-Purchase-pending-on-Steam
It should now work.