Cause

This message indicates that someone changed the default login shell for root to a program that is missing from the system. For example, the final colon-separated field in /etc/passwd could have been changed from /sbin/sh to /usr/bin/bash, which does not exist in that location. Possibly an extra space was appended at the end of the line. The outcome is that you cannot login as root or switch user to root, and so cannot directly fix this problem.