I had the same error message a few months ago so I understand your pain!
Unfortunately the error message is misleading because issues with MSFS, Windows, or Xbox can cause this message to occur. In order to fix this problem, you may have to try different things which is frustrating and time consuming. There is a MSFS Marketplace Purchase FAQ that lists steps to follow depending on your MSFS installation that should eventually fix your problem. It is important to follow the steps in order and do not skip anything.
I skipped one of the steps and it took me about two weeks of backtracking to find and correctly fix the error. It is tempting to reinstall MSFS or Windows. These are a waste of time because the reinstalls don’t work. (One Microsoft - not Asobo - recommendation was to completely format every disk and install everything “fresh” without using backups. This isn’t in the FAQ and I did NOT do this.)
I was having this error message because there were two Windows updates that failed every installation attempt. I discovered that the code used to install Windows updates is used for Marketplace purchases. (Hey, I didn’t design this mess!) When I fixed the Windows updates installation problem, Marketplace purchases started processing normally.