ubuntu doesn't have the classic 'root' user, if you read the direction
on elinux where you got that image you would have first read, about
the 'serial only option' you need to pass thru "setup_sdcard.sh"
For Serial Port Users add: (to use default user/pass ubuntu/temppwd)
--use-default-user
and then you would have found the default user and pass as:
user: ubuntu
pass: temppwd
Regards,