Hello, there are a few results on the “Another transaction is pending” with some suggestions and FAQ here:
https://flightsimulator.zendesk.com/hc/en-us/sections/360004530679-Purchase
Search results for ‘Another transaction is still pending’ - Microsoft Flight Simulator Forums
If you are still having issues, you may wish to contact support on the MSFS Zendesk if you haven’t done so already. You can open a support ticket here for further assistance.
I’ve also moved your topic to #self-service:miscellaneous where purchases are discussed.
Hopefully this can be resolved for you soon!