It’s quite possible that “root” credentials may not have been generated during the creation of the VM such is the case with DigitalOcean if you choose to use a “public/private key” pair instead of a password.
Also, yes the installer DOES NOT provide these credentials which may seem odd for a noob as the credentials are the same as the ones created for the “system user”… That is, whatever credentials are presently being used for example “root” user to “SSH” into the system, are the ones used for the Virtualmin login.
*** Lets NOT assume newbie users know this, so lets soften the tone when we respond please. ***
Would it be a good idea to have the login script start with or end with a hint? Maybe offer to set up a user for those with keys only? I know my AWS account requires keys. I get logged in as some random word/number salad of characters.
Webmin and Virtualmin authenticate to system users. Login with your root user and password. If you do not have a root user, but you have a sudo-capable user, login as that user. If that user doesn’t have a password (e.g. you logged in with a key), you need to set one.
That thread does give a command to run to set a password: passwd
If you just run passwd without a username, it will set a password on the currently logged in user. But, if you don’t know your password, you’ll need to either become root with sudo su - or run it with sudo passwd
The root user can set a user’s password without knowing the existing password.