systemd doesn't start my custom service and logs "Cannot add dependency job ... Unit not found"

Issue

• Upon boot, my custom service custom.service doesn't start automatically, although enabled, and journalctl -b shows the following line

  systemd[1]: Cannot add dependency job for unit custom.service, ignoring: Unit not found.
  

• After booting, performing a systemctl operation on the service fails

  # systemctl status custom.service 
  Unit custom.service could not be found.