This feature is only available on Enterprise plans.
If you are looking for general Member Posting Permissions, check out this help article.
Managers have the ability to turn off all comments and private replies for all posts in a group.
Turning off Public Comments and Private Replies will allow only posts to be created and no comments by any member or manager in this group.
TIP: Pair this with turning off other post types to allow a “Community Announcements” group.