Best Tip: Follow the Manual.
But, I'm going to ignore that tip, in this answer.
Make sure the Syquest device id is 6, that the cable is connected correctly, the drive is on, there's a disc cartridge inserted and it's and in a ready state.
Make absolutely sure the Syquest disc drive is a SCSI type, and not a Parallel type of drive. The old style SCSI connector is identical to a Parallel connector, so it's concievable that the wrong type of drive could be plugged in.
Those are the most basic things.