So i have a 1975 280z 2+2 i cant shift into any gear what so ever when the car is on but when the car is off i can shift gears perfectly, could someone help me with this I've already replaced the salve and master clutch cylinder.

Did you bleed the system after you installed the new parts?  You have to get the air out.

 

Also, tell a little bit about the car.  Was it working fine, then stopped?  Did you take it apart for some other reason, like a new clutch?  Has it been sitting for 20 years and you're just trying to get it running?  Did you just do a transmission swap?

 

Each of these can have a different solution.  Need details.