Board Game Arena

How does this work? Trying to buy a card and gain the extra 6 VP for adding a gold, and it won't let me...

You must have an available gold before getting the card bonus (you can not spend a gold granted by the card you just buy), and you should not have spend all your gold for buying the card (in most cases it's better to use just wood). The game then ask you if you want to spend a gold or not. Of course, Harsh winter option must be on (decided by the game table creator).