It is better to disable root user login since allowing root user login is a major security concern in Linux system. You can use root login after logging in as another user.
1. Edit sshd_config file.
sudo pico /etc/ssh/sshd_config
2. Search PermitRootLogin directive in that file and change PermitRootLogin to no. Example:
# Authentication: LoginGraceTime 120 PermitRootLogin no StrictModes yes