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.

-100000 Coins.

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.