By default you can not log in to Cockpit by root user.

You can fix this issue by editing the /etc/cockpit/disallowed-users file.

sudo vi /etc/cockpit/disallowed-users

Remove the root entry. (if needed add/remove other users if applicable).

Restart Cockpit service:

sudo systemctl restart cockpit.socket

Now you can access Cockpit as user root as well.