This happens to me all the time. This is what I have found the issue to be in my case. One of the elementary schools that feeds into my school uses code.org with students. The teacher there created a course and gave them the email sign-in option (rather than using picture or code word sign ins), so they used their school issued emails to sign in to code.org.
Unfortunately in my case, several students didn’t remember their passwords. But, when they tried to reset the passwords, it sent a reset email to their account and our district blocks outside emails, so I was able to resolve the issue by e-mailing email@example.com and they sent me password reset links for each affected account.
Then, once logged in, you can go to the Account Settings Page and find the button to link the account to he Google Single Sign in Option (the way I prefer my students to sign in).
I later found out that all students used the same password as per the instructions of their Elementary School Teacher, so I know that password now and can help them without contacting firstname.lastname@example.org
Hope this helps!