According to the detail info. This should be a license issue. For a specific account which have VS Enterprise not detect by VSTS and VSTS assigned him a default Basic license. VSTS have 5 free users which have default Basic license.
Based on the warning level and error message, this will not hinder your migration. After the migration finished, then you could deal with the particular account's license. Visual Studio Subscription linking is something that happens outside of the scope of an import. Usually VSTS will auto detect Visual Studio Subscriptions after the user signed in.
You could take a look at below explanation from MSDN:
TfsMigrator is unable to detect Visual Studio subscriptions (formerly
known as MSDN benefits) when generating the identity map log file.
Instead, it's recommended that you leverage the auto license upgrade
feature post import. As long as a user's work account is linked
correctly, VSTS will automatically apply their Visual Studio
subscription benefits on their first login post import. You're never
charged for licenses assigned during import, so this can be safely
handled post import.
You don't need to repeat a dry run import if users don't automatically get upgraded to use their Visual Studio Subscription in
VSTS. Visual Studio Subscription linking is something that happens
outside of the scope of an import. As long as the work account gets
linked correctly before or after the import then the user will
automatically have their license upgraded on the next sign in. Once
they've been upgraded successfully, next time you import the user will
be upgraded automatically on the first sign in to the account.
Source Link
If the user still get the license issue, you could contact our license support team.
