Flask: No user_loader has been installed for this LoginManager

[Truman]

and again I get

Error:
Exception: No user_loader has been installed for this LoginManager. Refer tohttps://flask-login.readthedocs.io/en/latest/#how-it-works for more info.

by the way the syntax from line 3 doesn't look correct to me after removing typo.
I'll try my luck with on facebook group.