The warning isn’t anything to be concerned about, that’s just because it’s your first time trying to execute script code and Blizzard wants to make sure you weren’t tricked into running shady stuff.
The command that you’ve shown here is safe, though the capitalization may be off, i’d use: /script FlipCameraYaw(180)
You can simply run it again to undo (as current + 180 + 180 = current + 360 = current).