cant open scripts

i have followed the tutorial video exactly, and am very new to game dev as a whole but when i click the script and clock open as is shown in the tutorial for space shooter just nothing happens and i get this error message: Unable to open Assets/Scripts/PlayerController.cs: Check external application preferences. is there any way to fix this? or did i miss something?

You need to setup your external script editor :

Edit → Preferences → External Tools

You can pick Mono Development if you installed it, or you can pick another editor of your own choice.