Permission denied when running any SUDO commands despite ALL commands are permitted

Issue

Permission denied when running any SUDO commands despite ALL commands are permitted.

• SUDO rule is in place to allow user to run ALL commands

  user1            ALL=(ALL)       NOPASSWD: ALL
  

• However, the user is not able to run any permitted commands via SUDO, with Permission denied error

  $ sudo su -
  su: failed to execute /bin/bash: Permission denied
  
  $ sudo visudo
  visudo: unable to run /bin/vi: Permission denied
  visudo: /etc/sudoers.tmp unchanged