Hello and Welcome to LinuxQuestions,
Ubuntu is against the use of the root account for security reasons, so they haven't set a password for it. The root account is not something to play around with, you should use sudo for your maintenance tasks. I'm not sure since I don't use Ubuntu but normally if you'd run
you get a root shell. If you really want to go into single user mode (do you mind indicating why?) then you could run
from that root shell.
Read why Ubuntu doesn't favor the use of the root account and password
here and
here. Have fun with Linux.
Kind regards,
Eric