I had the same problem with a brand new PC built for MSFS 2020, and a new install of the game.
I am now able to purchase addon products in MSFS 2020 Marketplace. I called Microsoft support, and they somehow determined that my base installation of Windows 10 was corrupt, so they walked me through a fresh installation of Windows 10 on my computer, at which point I was able to start making purchases in MSFS 2020 Marketplace.
First they updated Windows 10, then they “repaired” and then “re-installed” Microsoft Store and the Xbox app on my computer, but nothing worked. Finally, they walked me through a reinstallation of Windows 10, and that worked.
Probably not what you wanted to hear, but that is how they fixed my problem. So I would suggest you do the same in this order…
- Update Windows 10 to the latest version (install all updates available)
- “Repair” you Microsoft Store and Xbox apps
- Reinstall your Microsoft Store and Xbox apps
- As a final resort, reinstall Windows 10 to the latest version
Even though my Windows 10 seemed to be running perfectly, and my computer is only 2 months old, Microsoft support said it had somehow become corrupted in the interface with Microsoft Store and Xbox, and in the end, they were right.
Hope that helps!