@camila Nice problem description.
Did you try to google it?
I just entered this at google: “mandatory authentication must log into your Google account”
And got as result these two topics:
Search Result 1: https://www.androidpit.com/how-to-fix-google-play-authentication-is-required-error
Search Result 2: http://android.stackexchange.com/questions/20369/getting-authentication-is-required-when-trying-to-access-the-play-store
And a youtube video: https://www.youtube.com/watch?v=FeakFdvDFtI
The great part about google is that you get several answers and can pick one you understand.
Did you try all of those? And did you google before, are you now here b/c none of such guides works?
If so please write down what you already tried. So that we can try something else
Please ask questions if you have any.