"Key Not Detected" message in cluster and can't start vehicle in 2015+ Ford Mustang, after installing Sync 3 upgrade

"Key Not Detected" message in cluster and can't start vehicle in 2015+ Ford Mustang, after installing Sync 3 upgrade

All Mustangs 2015 year or newer are push to starts. The main antenna for the push to start is located in the center console. You'll run into this symptom if you unplug that antenna and don't return it before finishing installation.