Did you switch ON access for Less Secure Apps and are you still getting errors?
‘Failure: error login on [imap.gmail.com] with user [firstname.lastname@example.org] auth [LOGIN]: 2 NO [ALERT] Please log in via your web browser: https://support.google.com/mail/accounts/answer/78754 (Failure)
To protect your account, Google might make it harder to sign in to your account if they suspect it isn’t you trying to export your email. For example, Google might ask for additional information besides your username and password if you are traveling or if you try to sign in to your account from a new device. If this is the case browse to https://g.co/allowaccess and follow the instructions. When done try to start the migration from your Soverin Dashboard.