On-premises versions of SharePoint have an audit feature that ShareGate's migration tool can access to generate an audit report. This report gives you a breakdown of various user actions on your sites.
You see this error message when you try to see the results of a scheduled Audit report for your Microsoft 365 environment:
The ShareGate migration tool's Audit report does not support Microsoft 365 environments
Microsoft phased out the classic auditing feature in SharePoint online in favor of a unified audit Log. The unified log can encompass other Microsoft 365 elements, such as Teams and Exchange.
The unified audit log is not currently supported in ShareGate's migration tool.