The front derailleur does more than shifting the chain from ring to ring, its secondary function is to keep the chain on the chainring. Modern chainrings are designed to facilitate shifting, so it is easy for the chain to fall off if there is no derailleur to keep it there. There are chainrings designed for single speed use that do not have the shift gates used for multi speed cranks. So, you can install a front derailleur or you can change your chainring, your choice