By default operators does not have permission to delete his own chats. You grant them permission to delete his own chats by assigning.

"Allow user to delete his own chats" this function to operators role (deletechat).

After you update role. Operator have to logout and login again.

If you want operatos permission to delete all chats you have to give them permission.

"Allow to delete all chats" this function to operators role  (deleteglobalchat)