pam.d/su to restrict su access. SU Module Unknown Error

Hello All,

I am trying to configure my server(Ubuntu 16) to allow one of my users to only su to one user. Like "su - tadess" will work since it is allowed, but "su - root" will fail since it is not on the allow list.
http://blog.serverbuddies.com/how-do...of-su-command/ is the link that I've used.

Now I'm getting the error "su: Module is unknown".

Below is a copy of my /etc/pam.d/su file. Blue is what I changed.

# The PAM configuration file for the Shadow `su' service
#

# This allows root to su without passwords (normal operation)
auth sufficient pam_rootok.so

# Uncomment this to force users to be a member of group root
# before they can use `su'. You can also add "group=foo"
#Located in /etc/pam.d/su
# to the end of this line if you want to use a group other
#
# than the default "root" (but this may have side effect of
# denying "root" user, unless she's a member of "foo" or explicitly
# permitted earlier by e.g. "sufficient pam_rootok.so").
# (Replaces the `SU_WHEEL_ONLY' option from login.defs)
auth required /lib/security/pam_wheel.so group=keethulrich

# Uncomment this if you want wheel members to be able to
# su without a password.
# auth sufficient pam_wheel.so trust

# Uncomment this if you want members of a specific group to not
# be allowed to use su at all.
# auth required pam_wheel.so deny group=nosu

# Uncomment and edit /etc/security/time.conf if you need to set
# time restrainst on su usage.
# (Replaces the `PORTTIME_CHECKS_ENAB' option from login.defs
# as well as /etc/porttime)
# account requisite pam_time.so

# This module parses environment configuration file(s)
# and also allows you to use an extended config
# file /etc/security/pam_env.conf.
#
# parsing /etc/environment needs "readenv=1"
session required pam_env.so readenv=1
# locale variables are also kept into /etc/default/locale in etch
# reading this file *in addition to /etc/environment* does not hurt
session required pam_env.so readenv=1 envfile=/etc/default/locale

# Defines the MAIL environment variable
# However, userdel also needs MAIL_DIR and MAIL_FILE variables
# in /etc/login.defs to make sure that removing a user
# also removes the user's mail spool file.
# See Idea in /etc/login.defs
#
# "nopen" stands to avoid reporting new mail when su'ing to another user
session optional pam_mail.so nopen

# Sets up user limits according to /etc/security/limits.conf
# (Replaces the use of /etc/limits in old login)
session required pam_limits.so

# The standard Unix authentication modules, used with
# NIS (man nsswitch) as well as normal /etc/passwd and
# /etc/shadow entries.
@include common-auth
@include common-account
@include common-session