Is the gearshift knob floor mounted? If so, try hitting it downward with your fist. I’ve seen this happen in other cars. The shifter knob gets slightly raised-up, car “thinks” it’s in gear, but it’s not. Good luck.

One new tire coming up!

Ask your mechanic friend what a thermostat jiggler valve is. It’s part of the thermostat. Google it. If he doesn’t know, stay away.