My car will not turn off after the ignition has been turned off. Dash screen gives message "shift to park". Dealership can't find the problem

I took my 2010 Buick LaCrosse to the dealership several times in the last 1 !/2 years. They kept telling me they could not make it show the same message. Finally the car would not go into any gear. Thank goodness, it happened in my driveway. The dealer replaced a module. He said that would take care of it. Less than two months later---same thing started happening. Again, they said it would not give them error messages. I told them to put it on a slight incline (up or down) when they parked it and it would say "Shift to Park" again. One hour later, he calls me back and said it was the shifter module--that it would cost me $420. The car has 87650 miles on it---he said my warranty would not cover that repair. I bought a Buick because they had a reputation for reliability---from my experience with buying this vehicle new and taking care of it----this reputation is not the same any more.

I have a 2010 Lacrosse V6 and I surely won't brag about anything about it or the dealership. The promise of good mileage at 13.8mpg is less that I got with 2006 Cadillac DTS. There have been more than five times there have been electrical problems and the dealership told me that they couldn't determine the problem if I COULDN'T DUPLICATE IT. I wish I had my Cadillac back! GM discontinued the wrong cars IMOP

Where are the answers?