Beginning with Kali 2020.1, the superuser account no longer exists, and the default user is now a standard, non-privileged user. If you would like to use root instead of the none superuser account on Kali Linux, here are the commands&instructions:
Issue command ?sudo su?
<Enter the password for kali user account>
Issue command ?passwd root?
<Enter the new password and retype that password again>
Now, your root password changed, try to re log-in or you can just switch the user and log in as root.