I had the same issue. I migrated my account yesterday and was not able to log in today. I have no idea how we were supposed to know to push "go back" and add account. There's no reason to even think "go back" would do anything since the login page is where you start when the launcher starts. I haven't been anywhere else to go back to.
In all the FAQs on migration and searching the internet for help, I never found anything saying to do that. I spent several hours trying to fix the problem and even resorted to emailing Minecraft support about the issue. They just told me to contact Microsoft since it's a Microsoft account now.
It wasn't until I read your post that I figured it out. So thanks!