Unable to assign OFF shift

Summary

Unable to assign Off period in case of daily changing shift

Content

Hello experts,
Our client does not follow any defined shift/schedule (like standard M-F) or OFF periods. They work in different shifts daily. And similarly their OFF period is also not fixed.
So, we are assigning daily shift from 'manage work schedule assignment' and it works fine. I am looking for a way to assign OFF shift.
I was trying to assign 'OFF Period' through resource exception, but I get the error saying "The dates of exception must be within the start date and end date of the schedule assignment. Please see the document for screenshot