This error is due to the switch to OAuth2. Google only supports OAuth2 authentication, which requires an OAuth2 challenge.
An OAuth2 challenge requires user actions in order to authorize MigrationWiz to access their data. After the following steps, MigrationWiz will send an email to each user mailbox, to ask for access privileges.
If you want to skip this manual step, you can use Administrative authentication instead, by following the directions in the What is OAuth2 Challenge? article.