If you are using eveng then the pre-entered password is eve.

Can you login to your server with the root account using any SSH/SFTP client at all?

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:
https://linuxize.com/post/how-to-change-root-password-in-ubuntu/

I use WinSCP V5.17.10， tks

Submit screenshot

I have a problem logging into WinSCP

1. host name is IP address of development board
   
2. port number is 22
   
3. user name is root（root is wo set name）
   
4. password is wo set inUbuntu；
   

When I log in, you are required to provide a password entered in advance. But what is the Pre-entered password？ How can I log in WinSCP？

tks