Comments are stored as nodes under the the document node. So if your user doesn't have the permission to create a child, it's expected that he can't comment. Actually he shouldn't even see the "add comment" section at all. What interface are you using?
<blockquote>Which permission I need to add to allow user to add comment but not allow to upload content ?</blockquote> Unfortunately you can't do that with the default web applications provided by Alfresco. You should add your custom logic to this apps for changing this behavior but this means changing the point of view related to the manipulation of contents. Actually these applications are content-centric so this means that a specific permission is related to the content and all the stuff around the content itself.