It looks like you’re really close to getting it working. I think it’s just a permission issue.
Looks like the video recommends the “linuxserver” duplicati docker image instead of the official project image. The linuxserver one takes an arguably better security posture by not running duplicati as root, but this causes some complication when it comes to permissions.
Make sure whatever user the duplicati docker container is running as has write access to the backups folder (where ever it is mapped on your host machine).