The file /etc/securetty comes with self explained header "# /etc/securetty: List of terminals on which root is allowed to login."
And the content is as follows
# For people with serial port consoles
#ttyS0
# Standard consoles
tty1
tty2
tty3
tty4
tty5
tty6
tty7
I just need to uncomment one line. My problem is solved now. Thank you for help.