By default all the users can post their comments on knowledgebase articles. Further, permissions can be enabled for the comments section, so that it would be visible to the users belonging to the allowed user groups only. The permissions can be enabled from:
Admin CP > Users > Manage User Groups > "Select User Group" > Permissions Tab > Core > Can Post Comments.
Additionally, the permisions need to be allowed for posting comments for knowledgabse category as well as articles. For categories, permissions can be enabled from Staff CP > Knowledgebase > Categories > Options Tab > Allow Comments.
For articles, the permissions to allow comments can be enabled from:
Staff CP > Knowledgebase > Manage Knowledgebase > "Select Article" > Options Tab > Allow Comments.
The permissions for articles will overrule the permissions for Category.