If the email was issued some time ago then the link could well be invalid as they do time out after a period. If you check the Timesheet Reminders area under the Client Reminders the timesheet will probably be there and you can issue a fresh link by sending a reminder to that authoriser:
Finally, check the authoriser is using the most recent email, especially if you have sent out reminders.
Please see the following article for more help with this problem:
We have an authoriser who when they click the Etzsign link it says it's invalid, even though we have sent timesheet reminders to them?
If they are having problems accessing the Etz system then please refer to the following knowledge base article:
Internet Browser Security Settings