OK, so you are using an encrypted partition!?!?
But, no matter, because I think we are closer to the problem now.
If you need to execute a command as root then
sudo whatever-the-command-is
and you will be prompted for your password.
If you need a login shell running as root then
sudo -i
You can’t out of the box do anything that requires root’s password because the root account is disabled, so best to ignore the su command. Refer: Expired root account