Sharegate displays the following error :
Office 365 was unable to create the Microsoft Azure migration job.
This error occurs in Office 365 when the creation of the Azure migration job failed. The most common reason for this error is because the Migration API functionality was not deployed on your tenant by Microsoft or is behaving incorrectly.
The solution to this issue is to contact the Microsoft's Office 365 support team to discuss this issue.
Before contacting Microsoft, you can troubleshoot this issue by testing if the same error occurs in another site collection and in another library in the same site collection. Another useful piece of information you can provide them with is that when using the API's "CreateMigrationJob" function, the resulting GUID is empty. Don't worry, it should make sense to them :)