Unable to connect to SharePoint Online using ADFS and manual authentication.
Sharegate only supports connecting to SharePoint Online (Office 365) with ADFS if Windows Integrated Authentication is used.
1. If you use another authentication scheme, an easy workaround is to create a new account in the cloud that is not provisioned through ADFS, such as firstname.lastname@example.org.
2. Try to connect to your site using the "Browser Authentication" mode.