EndeavourOS comes with picom. For this to work you either need to use picom-git or if you want to use advanced features…one of these:
https://aur.archlinux.org/packages/picom-tryone-git
https://aur.archlinux.org/packages/picom-jonaburg-git
https://aur.archlinux.org/packages/picom-ibhagwan-git The script you linked uses this one.
The three above depending on what you are trying to accomplish may need this in ~/config/i3/config:
exec --no-startup-id picom --experimental-backends -f ( or -b) you need to figure out what is best for your use case.