Just to provide some background.
The root account on Raspbian (most Debian based distros actually) purposely has its ability to login or su to disabled. Said another way, you can’t log in as root. There is no root password. You are expected to log in as a regular user and sudo to perform actions as root.
If you want to allow someone to log in as root (not something I’d recommend) you have to give root a password and, potentially on some distros, give root a shell as it may default to /bin/noshell or /bin/false (I’ve never seen this but it is in the realm of the possible).