Open a Command Window
Input command: sudo passwd root
Then follow the steps in.
The differences between the Su and SU commands:
The most essential difference between the SU command and the Su-command is that the former simply switches the root identity, but the shell environment is still the shell of the normal user, and the user and the shell environment are switched to root. The PATH environment variable error does not occur unless you switch the shell environment. After the SU switch to the root user, PWD, found that the working directory is still the normal user's working directory, and after the Su-command switch, the working directory becomes the working directory of root. Use the echo $PATH command to see how the Su and Su-future environment variables are different. And so on, to switch from the current user to another user, you should use the Su-command.
See details:
It is also important to note that the Su-command has a space before Su and-
How to get root permissions for an Ubuntu general account and the difference between Su and Su