An error happens when migrating because one or more features are not activated at the destination. You will get an error message like:
The feature failed to activate because a list at 'Documents' already exists in this site
Could not activate feature because of the following error: There can only be one instance of this list type in a web.An instance already exists.
This error occurs when a feature is required for the migration to execute properly or to warn that a feature does not exist or is deprecated (for instance, the "Translation Management Library" feature) at the destination.
For existing, non-deprecated features, activate them at the destination and restart the migration. See how to activate features with Sharegate.
To activate features from SharePoint, go on your site collection or site settings and click on the "Manage Site Features" or "Site Collection Features" link.
Once you're on the Features page, you can activate/deactivate all the features you want by clicking on the Activate/Deactivate button.