The creator of the script must be Code Violet or it won't work. They even say so on the GitHub page.
Outlined here: https://avsitter.github.io/avsitter2_prop.html
Second Life has now enabled ‘experiences’ which will allow automatic attachments (without need for any permission request).
Important: The packaged version of AVsitter (inworld updates and marketplace) includes the [AV]object script compiled with the AVsitter experience by Code Violet. If you compiled [AV]object from GitHub yourself, this section doesn’t apply, as the script won’t have the experience enabled.
For automatic attachments to work, Users will need to enable the experience ‘AVsitter’ (created by Code Violet) on their land. An experience may be enabled at the Estate or Parcel level by adding it to the ‘Allowed Experiences’ list. For an excellent explanation, see this video by Froukje Hoorenbeek. Be sure to choose the experience created by Code Violet as anyone could have accidentally created an experience with the same name.
You can find out more about ‘experiences’ on the SL blog and in the SL scripting WIKI.
Important: If users do not enable the ‘AVsitter’ by Code Violet experience then each