The quest is asking you to buy and place a certain item, but when you check the menu you see that item is locked - you've already place the maximum amount in your cafe.
No more Black Kittens allowed!
To workaround this, sell one of your placed items back to the menu and buy a new one again. You can do this by dragging the item into the menu and then finding it in the Warehouse.
Click "Sell" to sell back the item.
Remember: you must spend Coins for this task to be counted. You will know if Coins were spent when you see Coins being subtracted when you place an item.
If you do not want to sell one of the items back and buy a new one again, you can use Cafeland cash to skip this task on any quest.