When attempting to copy an item, the ShareGate migration tool displays the following error:
Sharegate was unable to find the user "username" through the people picker. This can happen if you are using a custom people picker. Please ensure that you are using the out-of-the-box SharePoint people picker.
When resolving users for permissions or metadata, the ShareGate migration tool will try to find the users on your SharePoint server with the SharePoint people picker. The people picker is a built-in page you can use to search for users in your SharePoint environment. If the people picker page is modified too much, the application will be unable to use it effectively, and you will get this error.
Validate that you are using the out-of-the-box people picker.
If you seems to have the out-of-the-box people picker, validate that you are able to use the people picker on your SharePoint server.
Note: A simple way to test this, is by adding a user in a Person or group field in any list or library.