All users can delete their own comments. Additionally, if users create a task, folder, or project, they can delete other users' comments on it. Deleting comments may be restricted by the user's access role.
You can delete your own comments that you leave on tasks, folders, or projects as well as comments that other users leave on tasks that you created. Once you delete a comment it completely disappears from the stream of that task, folder, or project as well as from the global stream.
The comment is immediately deleted.
You can also delete comments from the Stream.
It's most likely that your access role in the current task, folder, or project restricts you from changing comments. You can ask the owner of the work item to update your access role or delete the comment for you.
The same goes for deleting other users' comments in tasks, folders, and projects that you created. Your access role in the parent work item or the whole space might have been changed and the ability to delete comments was restricted.