When you set up Kali (I have Kali running here - and in a Virtual Machine), it will ask you to create a password for the root account. Do that and that will be the password. There is no preset password.

Try setting up the Virtual Machine again and this time, watch for the root account creation process and set up the password of your choice.

Once running, create a regular account and, if you wish, set it up as an admin account:

sudo useradd -m YOUR_USERNAME
sudo passwd YOUR_USERNAME  # (asks you to create a new password and do that)
sudo usermod -a -G sudo YOUR_USERNAME

Just encountered the same problem with a fresh Kali Linux install. I chose to disable root login, and create a user account instead. Don’t remember giving a root password

But here’s an easy workaround to jump straight into a root-shell without the password:

sudo su -

simple as that, now you’re free to change your root password:

passwd