Let's check that Steam Overlay is enabled! It's required to purchase gold in-game.
1) Click on Steam and click the "Settings" button. Click the "In-Game Settings" tab.
2) Check the box next to "Enable Steam Community In-Game" and click OK.
3) In your library, right-click Orcs Must Die! Unchained.
4) Click "Properties."
5) Check "Enable Steam Overlay while In-Game" in the "General" tab and click OK.
If you've checked both of these options and are still having trouble, you might be experiencing a bug.
If this does not resolve the issue, there might be something else happening. Please check our article on on Steam purchasing here.