GM-GMC: transmission, hydraulic clutch, clutch disc

Question
transmission will not go into gear when engine is running but will go into reverse. Transmission will shift into gear when engine is not running. I just replace the clutch kit. Thanks for your help.
1993 chevy pick up, 6.5 liter diesel, duelly, 1 ton.

Answer
I believe that your vehicle has a hydraulic clutch.

It may have air in the line.  I also understand that these vehicles have a hard to bleed hydraulic clutch.  You might have to move the vehicle such that it is nose up or nose down, depending on the clutch line angles, to help get all of the air out.

The other possibility is a clutch disc installed backwards.  The center of the clutch has a flat side and a side that houses the take-up springs.  The flat side must face the flywheel.