Some items need to be built first in order to be used. The indicator of a buildable item is an orange hammer right next to the picture in the decoration menu. When you see a buildable item, you can tap on the "i" icon to see how many vouchers you need to build that item.
Please note that buildable items come in a box which can't be stored or sold. So we strongly suggest making sure you have enough space in your cafe to keep the items until you build them!