Hi Tim,
First thing:
In this screenshot from your video I can see that you are missing the end of the script. This is the part that actually creates the grid. See my code snippet above and note the difference.
Secondly you need to give your div block an ID (different to a class name) of myGrid. To do this you go to this part in the Webflow designer:
This ID is what the script uses to reference the div block. Try that and see how you get on.