How to troubleshoot authentication failure when su is entered in Ubuntu

When the ubuntu service is started, the system prompts that the permission is not enough. Use su to switch, and enter the password to prompt that authentication failed. Now, after checking the original Ubuntu installation, by default, the root user is locked. login is not allowed, and su to root is not allowed. for desktop users, this may be for enhanced security, but it is not feasible for the server version, set it up.

Terminal

Studiogang @ studiogang :~ $ Sudo passwd

Password: <--- enter the Password of the user during installation

Enter new UNIX password: <--- new Root User password

Retype new UNIX password: <--- repeat the new Root User password

Passwd: the password is successfully updated.