The root password has nothing to do with WinSCP. WinSCP does not have a password of its own. It only uses the password you use to access the files e.g. on a website.
The root password is the one used to access your actual device (usually Linux devices), e.g. if you have a VU+ satellite receiver and need to change something in the files using FTP.
This is what I found for you after a search:
By default, ubuntu has no root password set.
How to change root password in Ubuntu
- Type the following command to become root user and issue passwd: sudo -i. passwd.
- OR set a password for root user in a single go: sudo passwd root.
- Test it your root password by typing the following command: su -
There is a good article here with explanations and instructions: