Google has recently added a feature that blocks sign-in attempts from some apps and devices, which can cause the authorization error. Maybe this is what is causing your error. You will want to log into your Gmail account, then visit this link to find the Less secure apps page, and click on “Turn on” to grant access.
You can also try starting the migration from a different browser (Chome/Firefox/Edge/Safari) or update the one you are using if you have not already done so.
I attach an article that may guide you through the browser updating steps for each of the different available browsers.