It still will not let me start it in park or neutral 06 Buick leucern

Try keeping key turned to start while moving shifting lever back and between reverse and park. If it almost tries to start the problem is probably the neutral safety switch. Even If it does not it still maybe the switch.

Transmission range sensor , through electrical signals to the PCM ,it knows what gear the vehicle is in . Some all so call it a Park neutral switch , but it does more then that . The park neutral part of it won't allow the vehicle to start unless it is in park or neutral .

I had a a problem simliar to this that made it seem like it was the shift gear was the ignition switch,

If you start it in neutral, the auto locks don't auto lock. As an mechanic, I start in neutral out of habit because it technically reduces wear by avoiding going through reverse.shift out of park while in acc to nuetral and start

When parked on an incline, engage emergency brake FIRST, then shift into Park. Hope this helps! Works for my Buick.

If you can start it in neutral your linkage might need to be adjusted or your neutral safety switch could be going bad or need adjustment.

Remove the connectors and probe them with an ohmmeter.
Connect one lead of your meter to the terminal with the yellow wire and the other lead to the terminal with the purple wire. Set the parking brake and shift the transmission through the gears. When in park or neutral, the meter should read zero ohms.

this is either the neutral safty switch or it is the transmission linkage needs to be adjusted

You most likely have a faulty gearshift position start interlock switch.
Check / replace faulty shift switch.

I would first check to see if you have the proper transmission level of fluid. From your discription, it seems that the fluid is low. If it is full, you have some more major issues. Hope this helps you, Rick