How to fix : 'Username' is not in the sudoers file.

 To make user as sudo user we need to use below:     

$ su
> Enter root password: *******
$ visudo -f /etc/sudoers

Find the following section of /etc/sudoers file and add your users privileges:

# User privilege specification
root    ALL=(ALL:ALL) ALL
user_name ALL=(ALL) ALL

make a backup of the sudoers files as follows:
# cp /etc/sudoers /etc/sudoers.orginal

# visudo

Add the user like below:

#
# This file MUST be edited with the 'visudo' command as root.
#
# Please consider adding local content in /etc/sudoers.d/ instead of
# directly modifying this file.
#
## Next comes the main part: which users can run what software on
## which machines (the sudoers file can be shared between multiple
## systems).
## Syntax:
##
##      user    MACHINE=COMMANDS
##
## The COMMANDS section may have other options added to it.
##
## Allow root to run any commands anywhere
root    ALL=(ALL)       ALL

## Allows members of the 'sys' group to run networking, software,
## service management apps and more.
# %sys ALL = NETWORKING, SOFTWARE, SERVICES, STORAGE, DELEGATING, PROCESSES, LOCATE, DRIVERS

## Allows people in group wheel to run all commands
%wheel  ALL=(ALL)       ALL

## Same thing without a password
# %wheel        ALL=(ALL)       NOPASSWD: ALL

## Allows members of the users group to mount and unmount the
## cdrom as root
# %users  ALL=/sbin/mount /mnt/cdrom, /sbin/umount /mnt/cdrom

## Allows members of the users group to shutdown this system
# %users  localhost=/sbin/shutdown -h now

## Read drop-in files from /etc/sudoers.d (the # here does not mean a comment)
#includedir /etc/sudoers.d
tonu ALL=(ALL) ALL
oracms ALL=(ALL) ALL

Setting Correct Permission on sudoers File

# chmod  0440  /etc/sudoers