Email approval : "The message you sent appears to be Un-solicitated"

Summary

User receives an error when approving absence via email that stats the email is un-solicatated

Content

Hello

One of our users has attempted to approve an absence via email - they have clicked the approve link which has open a the reply email and sent it, they received the below message back;

The message you sent appears to be un-solicited. If you are responding to a notification, use the response link that was included with your notification.

Invalid action on workflow task or user does not have privilege to perform this action. Action APPROVE on task a01197a3-332d-414b-8046-cd326e2e31f4 cannot be performed by