Slots that require coins/gems look like this:
Contrary to slots that require items, gems and coins cannot be dragged towards the slots one by one. The player needs to have the full amount or more in their resources bar at the top:
If the player has the exact amount or more, they can go back to their inventory and select the coin/gem slot they wish to unlock.
Then at the top, the following interface will be visible:
The player then needs to click on the green button to unlock it. If the required amount is however missing, the button will remain greyed out:
