Use sudo -i passwd root

But, ubuntu has an additional security layer to block root account login.
We should change some files, but take backups of them for any future problems.

Use mkdir backup
then cp /etc/gdm3/custom.conf backup/
then cp /etc/pam.d/gdm-password backup/

Check using ls backup/
if there are custom.conf and gdm-password in backup, it is a success

Now, use sudo nano /etc/gdm3/custom.conf
Type AllowRoot=true between [security] and [xdmcp]
Save it using ^o and enter then ^x

Now, use sudo nano /etc/pam.d/gdm-password
Here, put # in the third line before auth required pam_succeed_if.so user !=root quiet_success to make it as a comment, and save it.

Now, reboot using sudo reboot -f

To disable root password and lock it again, use sudo passwd -dl root
Then, restore original backup files of custom.conf and gdm-password
 sudo cp backup/gdm-password /etc/pam.d/gdm-password
 sudo cp backup/custom.conf /etc/gdm3/custom.conf
Or, remove AllowRoot=true form custom.conf and # form gdm-password

Refer to:
https://www.computernetworkingnotes.com/linux-tutorials/how-to-enable-and-disable-root-login-in-ubuntu.html#:~:text=Enabling%20and%20disable%20root%20login%20in%20nutshell&text=CLI%20%26%20GUI%20both-,Use%20the%20sudo%20%E2%80%93i%20passwd%20root%20command.,conf%20file