Newly installed ubuntu default can not use the root, we need to manually open.
In the terminal type: # sudo passwd root
[sudo] password for gaavv: 123456
the Enter new new UNIX password: (enter your password on this) 123456
the Retype new new UNIX password: (OK you enter the password) 123456
passwd: password successfully Updated
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Later, if you think to get root access, just the following operations:
#su root or #su - // into the root
Password: (Enter the password you set above)
#exit // exit the root