Both Steam and Marketplace installs need a functional Gaming Services and XBox apps installation.
I would first try resetting Gaming Services and Xbox from the Windows Apps and Features menu.
There is a prompt in each for Advanced Options, the reset buttons are in there.
Restart the PC and sign back into them when you are done.
You can also try signing out of Steam, then restart the PC and sign back in, and then try the sim.
If these don’t help, you can try removing the files involved from the MSFS Official-OneStore folder (don’t delete them, cut and paste them somewhere else, just in case) then start the sim and see if it will reinstall these.
It should.
If all the above fail, you can also enter a Zendesk “Submit a Request” from the “Support” menu at the top of any forum page.
Bear in mind it can take up to a week for them to reply at times.
Let us know if you are still having issues after all this!