Unable to set user as the site collection owner

Symptoms

When attempting to migrate a site collection to Microsoft 365, Sharegate shows the following message :

The user <USER> couldn't be set as the site collection owner. This is most likely because the user is unlicensed.

Error code

Details

When migrating a site collection, Sharegate uses the source site collection owner as the new owner or the site collection. If the user is unlicensed (Microsoft 365 only), the user performing the migration will be set as the site collection owner.

Solution

There are multiple solutions regarding this issue :

1. Use Map Users and Groups to map the source users who are no longer valid (or unlicensed) with users who are valid on the destination environment.
2. Attribute a Microsoft 365 license to the source site collection owner user, so he could be set as site collection owner in Microsoft 365.