This question already has an answer here:
Last night I decided to upgrade my phone to Android 6.0 and noticed I am unable to log into the app anymore. I have always used Google as my login for the site and I was logged into the app just yesterday, before upgrading to 6.0.
I double checked that I don't have a password associated with my account by using the "forgot" link and did get back a confirmation email saying that I don't have a password associated with my account and that I can login using my Google account.
Yet this is what my login screen looks like on my 6.0 phone.
And on my 5.1.1 tablet it looks like this.
Looks like a new permission model issue causing the Google login not to show up on Android 6.0, and because I do not have a password associated with my Stack Exchange account, I am unable to login.