There's difference between su
(switch user) and sudo
(superuser do). Both are different so don't confuse with them.
However, once login as root user and edit /etc/shadow
file. Remove encrypted password for root user i.e. 2nd entry against root user (marked in red) leave it blank as:
Save and close the file and then invoke passwd
command to reset root's password.
Then again login as any normal user and try su - root
or su -
and use new password for login.