It’s telling you the error, “Input Axis horizontal not setup”.
You don’t have an axis called ‘horizontal’ in your input, so unity can’t execute that line of code.
If you go to the input manager you can see all your axis(what ever the plural for that is), notice that the one listed there by default is ‘Horizontal’, with a capital H.
next time when you have an error, don’t link us to the tutorial, we’re not gonna watch that.
post the script you’re having trouble with in code tags with the full error message along side.