I get the same problem when trying to purchase the in-game editor for FM2017.
It's there on the in game menu screen, I add to my cart and try to checkout but I get the same error.
It's a shame you can't purchase it anymore, can't understand why either.. I get they can't sell old versions of the game due to licenses expiring. But dlc content like an in-game editor shouldn't cause any problems if it was still purchasable, plus SI could still make money from older versions. Not alot I know, but every little helps