It could be the administrator thing. If you run MSFS using ‘Run as Administrator’ it can prevent purchase from the marketplace. You get that ‘purchase failed’ notice in your profile at the top right of the screen.
It might just take logging out of MSFS and then opening it again without using the Admin. step.
Conversely at least one other user could only buy from the marketplace if he WAS logged in as Admin…so maybe it has to do with how you first ran the software or whether you installed and configured MSFS as Admin…