I agree with Ixoye find the soucre of the item you want to buy... better support and faster updates....
Ok after weeks of looking for a the fix finnaly
If you have the Steam version: The Steam, Settings, In Game, Enable the Steam Overlay checkbox is not checked. It must be checked to order from the marketplace.
Shout out to "Fielder" from Avsim for the fix it took me weeks until I found his post