You get the following error in the Migration report:
This item was not copied due to a fatal error on an other item. Check for items with error in this report
You can only get this error when you perform a migration to Office 365 in Insane Mode.
You receive this specific error when the application sends a package to Azure that gets more than 100 errors.
When a package reaches 100 errors, Azure cancels the import process.
The items that were imported in that package can be found in the Migration report with the actual error.
The items that got cancelled by Azure will have the This item was not copied due to a fatal error on an other item error on them.
Note: It's possible some of the items got migrated anyway, but ShareGate Desktop is unable to know which ones.
Review your Migration report to troubleshoot your errors.