What does the error Input Button is not setup mean?

I’m making a zombie apocalypse game and I got an error message that said: Input Button Menu Open is not setup. To change the input settings use: Edit ->Project Settings → Input
I’m sorry if the answer is really simple but I’m pretty new to unity. Also if possible it would be nice if you could think of the simplest answer.

In the top bar, the one with the FILE, EDIT, ASSETS, etc. tabs hover over the one that says Edit and then move your mouse and hover over the Project Settings tab and then click on Input. It will open in your inspector. In the field that asks for the size add one more to it (i.e. delete 9 and put 10). This will create another input option. Change the name of the last input option to Menu Open and then set the controls within.

Good Luck,