After installing a new thermostat the car keeps shutting off and reduced power message?

Hello, is the Check engine light on? The engine computer is keeping the vehicle in a reduced power/limp mode because it is detecting a problem that could potentially damage something, such as the Catalytic Converter or possibly the transmission. The vehicle should be scanned for any diagnostic trouble codes, a full system scan, not just the engine computer. But since you just did work on it, you should retrace your steps, you may have accidentally pinched a wire somewhere, or moved the wiring harness to where a connector is not making good contact inside now. Most likely the issue is around the area you worked. Check contact points of the wiring harness where it rests on a bracket or the engine block. Those areas are where the harness receives the most engine vibration and can rub through the conduit protecting the wiring harness. Or if you pulled on any section of the harness to move it out of the way to replace the thermostat.

https://www.2carpros.com/articles/checking-a-service-engine-soon-or-check-engine-light-on-or-flashing