The code you have would flip the player when it hits the camera and not the camera. You need to rotate the camera and not the player.
Try this:
function OnTriggerEnter(otherGuy : Collider){
if(otherGuy.tag == “cameraflip”){ //you don’t need to use .transform.tag .tag works just fine
Camera.main.transform.rotate(0, 0, 180); //Camera.main is the same thing as GameObject.FindGameObjectWithTag(“MainCamera”)
}
}