This happened to me also - sort of. I could click on the buy or buy and download options but got a message ‘purchase failed’ etc…
Turns out the problem was that I had started opening MSFS under ‘run as administrator’ at some point and THAT was preventing me from using the Marketplace. Because I had originally opened the sim without taking that step, now that I was the connection was failing.
Once I closed MSFS and then opened it again normally, without running as admin I could again make purchases in Marketplace
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