Ok. When you send the Approval card, it has a comments box for the approver to respond. Please refer below...
This box also appears within outlook.

These are optional comments and at the moment, we do not have the option to convert the field to required. However, if this is a concern, you could check if the status is rejected and comments are empty within your flow, and reassign the card back to the approver until such time that the approver enters comments.