My board is now full and can’t merge items….. I don’t have enough gems to buy a slot on the shelf..

Delete some of the items that have no use to you. Please remember that the boxes with the question sign "?" are used to help build items like grocery bag, coffee, fish, etc. I suggest once you maxed an item like coffee stop building other coffee machines. You can have like one as extra but its not worth the space on the board.

Sell/delete items. Click on it and down at the info box there should be a trash can or a coin.