The approval link that is sent in the Mail for service request approval, does not work . It gives this error:

Invalid Key : Key could have been changed manually or would have been an old one.

Why is this so ? The key has not been modified. Also, in the Notification Rules when I try to set the $ApprovalLink, there is no such variable in the column at the right that contains all the variables that can be included in the template..

Snapshot has been given below: 

Any solution to this?

Version and Build :   8.1.0  Build  8112

Thank you