The problem here is that the AvSitter version from GitHub does not support what you want it to do - work with the AvSitter Experience.
The paid version does and will allow prop attachments without asking for permission and is the only way to have AvSitter work with Experiences.
If I recall correctly this is because the GitHub version has the user as the creator while the paid version has the original creator as the creator of the scripts which is the crux of the matter. This affects the Experience and wether it works or not.
Edit: Did some digging and I was right.
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 prop will require permission to attach and the owner may receive the message “To enable auto-attachments, please enable the experience in About Land.” (discussion)