Update: I fixed my “Another Transaction is pending” problem. I tried all the steps ZenDesk gave me except for #1 - All current Windows updates are installed. I skipped this step because why wouldn’t my Windows be current? After wrestling with this problem for weeks, I checked my Windows updates. I discovered two updates from earlier this year consistently failed each installation attempt. The Windows Troubleshooter for Windows Updates didn’t find or fix the problem. I spent many days trying to fix this update problem. Most of the recommendations I tried had failed leaving as the final solution: “format everything, install Windows, restore your apps and files from backups” which was unacceptable to me.
After going through numerous Windows logs, I found one Windows dll file that was corrupt. I copied the dll from another Windows system to my PC. I restarted the PC and then retried the Windows update. The updates that had failed processed correctly!
Next I tried a purchase in the MSFS Marketplace. The MS Store purchase window opened and I completed the purchase!
If you are not able to buy from the Marketplace, check your Windows updates.
Even though the problem was 100% Windows, the Marketplace error message was completely wrong and misleading. Some users actually may have a pending transaction that is failing. I didn’t. My purchase attempts failed because of a corrupted Windows system.