I am trying to follow the instructions here for a headless install.
I have placed the ubuntu-21.04-live-server-amd64.iso image on a USB key on a machine that has no keyboard/monitor/mouse. I observed that it successfully joined the network and SSH is running. However, I cannot ssh into the installer. I have tried ubuntu:ubuntu and root:ubuntu, but both are not accepted. I have also tried ubuntu: (no password) per this question with no success. I also tried root: without a password and it is not accepted.
The instructions say:
If the only available terminal is very basic, an alternative is to connect via SSH. If the network is up by the time the installer starts, instructions are offered on the initial screen in basic mode. Otherwise, instructions are available from the help menu once networking is configured.
What are the instructions? What is the default username and password for the installer?