When attempting to provision a user's OneDrive site, Sharegate displays the following error message:
The user associated with the email 'firstname.lastname@example.org' cannot be assigned a OneDrive site.
The error occurs because the user associated with the given email can't be given a OneDrive site. This could be because the user is an external user or doesn't have the required rights to be given a OneDrive site.
If the user is an external user, he simply can't be given a OneDrive site. He must be added to the tenant before provisioning his OneDrive.
If the user isn't an external user, check the user's permissions to make sure he is part of a OneDrive-eligible group. If he already is part of a OneDrive-eligible group, try having the user provision his OneDrive site manually by logging in to https://tenantname-my.sharepoint.com.