I restricted the use of su by creating an admin group
sudo usermod -a -G admin "admin user"
sudo dpkg-statoverride --update -add root admin 4750 /bin/su
Later I regretted this so I removed the following line
root admin 4750 /bin/su
from /var/lib/dpkg/statoverride and I removed the admin group.
My /bin/su permissions:
-rwxr-xr-x 1 root root 44664 Jan 25 2018 /bin/su
When I try to login to root account with the new root account password I get
su: Authentication failure
I've no intentions of using the root account but I find this annoying. How do I solve this?