Why is my Mustang doing this!?

Can't make out what you're trying to say. You cant put it in park? You tried to start it while engine was running?

Sounds like the stupid security system going bananas . No one wants to steal your dumb car .

I have a vehicle do that in the past, it was a corroded plug and wiring associated with the ignition switch and lock cylinder. Your mechanic can find the issue in short order.