Hey guys I have an rx7 1991 2nd gen turbo, everything was good but when I turned the car on, it won't shift at all, whenever the cars off it shifts fine but whenever the cars on, it won't budge, I think it's the transmission but I'm open to other ideas

 

check shifter bushings? Also, your clutch might not be disengaging not allowing you to shift when the car is on.

 

Clutch master and slave cylinders 1st.

 

Forgot to reply to this, the problem was the clutch, not enough fluid was in the tank, thanks guys

 

This also sounds like a Pilot bearing problem..that is if your "clutch fluid" was not the fix you hoped for.

If the input shaft can't spin in the back of the engine's eccentric shaft and is virtually stuck there due to a bad bearing then it spins the whole Unit (engine and trans) as one..like it was in gear.
Your clutch is used as a "detachment" BUT If the pilot is bad the trans never detaches from the power of the engine as it won't allow the trans input shaft to spin freely.
You'd be able to select gears when the car is not ON but when you try to get the car to shift gears?...nope.