Unable to run su or ps command: Error: /proc must be mounted

Issue

• Unable to execute su and ps commands, getting the following error for non-root users.

$ ps
Error: /proc must be mounted
  To mount /proc at boot you need an /etc/fstab line like:
      proc   /proc   proc    defaults
  In the meantime, run "mount proc /proc -t proc"

or 

$ ps
Error, do this: mount -t proc proc /proc