While navigating through Service Portal in the "approval" page for a sysapproval_approver record for which the current user is the approver, the activity stream isn't shown or doesn't allow to insert comments/work notes at the bottom of the Approval Record widget.
The causes can be several and they are described below based on the scenarios shown in the screenshots while using the OOB widget Approval Record:
1) The user doesn't have read access to the approved record, i.e. change request:
2) The user doesn't have write access to the approved record:
3) The user doesn't have read or write access to the comments field of the approved record:
The same applies when work notes are not visible, it means that the user doesn't have read or write access to the work notes of the approved record.
In case, the scenario described in 2) is presented and the user have read and write access to the approved record it means that it doesn't have read or write access to both comments and work notes of the approved record.
Based on the scenarios described before the resolution is to either create a new ACL to give access to the user or modify the existing ones.