Assigned User for Knowledge workflow is not able to approve

Summary:

Hello All,

We are implementing knowledge management for HR Helpdesk. We enabled workflow for one of the content types and then after authoring the author starts the knowledge workflow. The workflow is set to auto-publish. But the assigned user is trying to approve the workflow task by navigating to Knowledge > Knowledge Tasks > approves the respective task, he is getting error like unable to approve the knowledge workflow task contact system administrator.

And how is the user who are shown while searching to assign the workflow step is being controlled? Like is there criteria for the user to appear in the assignee search box while the author tries to assign to other user for approval.