Some activities may show the text "Shared by: Admin Site" without an ellipsis (...) present for editing. This indicates that the activity was originally set up at a different site than the one you are connected to now.
The activity was set up in the Admin site and shared with the site to which you are now connected. This is done for the purpose of recording participation without having to set up this activity in each site, potentially saving considerable time, especially for larger districts with many school sites. This shared activity will be listed in this site's Activities section but cannot be edited. To edit such a shared activity, switch access to the site that shared the activity (in this case, the Admin site). To learn more about how to Switch Access, please see this article
If you do not have permission to switch access to the site where the activity was originally created and shared, contact your district system administrator to provide you with permission or ask them to edit the activity for you.