I am trying to disable comments from the story, referred document Disabling User Comments for Workbooks | Tableau Software
and was able to Deny Add/view Comments for users and group under Add/Edit Permissions dialog box.
The challenge is unable to find and perform point 9 (On the Permissions: Workbooks page, click Assign Permissions to Contents)
Request your guidance.
Make sure you're going to workbooks or projects then setting the permissions and not going to views. Under views you will not be able to assign permission to contents.
I hope this helps!