When attempting to copy a Nintex Workflow, ShareGate Migrate displays the following warning in the "Migration Report":

The escalation date for the action '{0}' cannot be converted. Please note that Nintex Workflow on-premises does not support setting the escalation date to something other than the current date.

When migrating from M365 to On-Premises, the parameter "Escalate On" does not allow the escalation date to be calculated from another date than the current date.

You need to check the box indicating to use the current date.