Overview

In the legacy UI, the Comments can be made mandatory on rejecting a 'sysapproval_approver'  record using a client script or UI policy. The user can achieve the same functionality by editing 'Approval Info' widget.This can be done by having a textarea in the HTML Body template of the widget and make it mandatory from the client controller script of the widget. 

Additional Information

Refer the below community post which explains the procedure to make the comments mandatory in Service portal.

Portal diaries: Service Portal — Making Rejection Comments Mandatory on Approval Record