If you receive the message "production queue is full" when the production slots of the building are empty, please make sure that you don’t have too many products ready on the building’s desk. If this is the case, you need to move these products to the barn before producing more of them, and this is the reason why you receive the message.
If your barn is full, you can upgrade it or you can sell some of the items you don’t need in your roadside shop. Once you cleared some space, move the products from the building’s desk to the barn and you will then be able to start the production once again.