If ShareGate detects an external sharing review recurrence that is fewer than 16 days or more than 999 days, one of the following messages will appear to alert you:
Recurrence must be at least 16 days
Recurrence can't be greater than 999 days
This usually occurs when you try to set an external sharing review recurrence outside the acceptable range.
Solution
The review recurrence (the amount of time to pass between each review) must be between 16 and 999 days. You cannot set a recurrence outside of that time frame.
From Policies, you can set the External sharing review policy and create custom external sharing reviews for sensitivity tags.
Update your review recurrence by following the steps in the following articles:
External sharing review policy - Set up and automate the External sharing review policy.
Custom external sharing reviews - Create and edit sensitivity tags.