So, select the script in the Project window. The Inspector changes to show you the script. Is the script Unity shows you identical to the script you posted above?
Add a Debug.Log to your Start() function. When you run the game does that Debug.Log write data to the console. (If so, then it means Unity has your script.)
Failing that, take a screenshot of your Unity window, post here, and we’ll try and work out what else might or might not be happening.