Network issue. Ifup says unknown interface

This is the output of ifconfig

# ifconfig
enp0s3    Link encap:Ethernet  HWaddr 08:00:27:ef:b6:e6
          inet addr:192.168.2.3  Bcast:192.168.2.255  Mask:255.255.255.0
          inet6 addr: fe80::cf19:776a:4d5c:36e0/64 Scope:Link
          UP BROADCAST RUNNING MULTICAST  MTU:1500  Metric:1
          RX packets:713069 errors:0 dropped:0 overruns:0 frame:0
          TX packets:6767 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:45869812 (45.8 MB)  TX bytes:667164 (667.1 KB)

lo        Link encap:Local Loopback
          inet addr:127.0.0.1  Mask:255.0.0.0
          inet6 addr: ::1/128 Scope:Host
          UP LOOPBACK RUNNING  MTU:65536  Metric:1
          RX packets:1108 errors:0 dropped:0 overruns:0 frame:0
          TX packets:1108 errors:0 dropped:0 overruns:0 carrier:0
          collisions:0 txqueuelen:1000
          RX bytes:86846 (86.8 KB)  TX bytes:86846 (86.8 KB)

When I do

# ifdown enp0s3

It says Unknown interface enp0s3

The network is fine and works without any issues. Its just the command that does not work

Ubuntu 16.04 (Xenial)

Below is the output of ip a s

1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
inet 127.0.0.1/8 scope host lo
   valid_lft forever preferred_lft forever
inet6 ::1/128 scope host
   valid_lft forever preferred_lft forever
2: enp0s3: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc pfifo_fast state UP group default qlen 1000
    link/ether 08:00:27:ef:b6:e6 brd ff:ff:ff:ff:ff:ff
    inet 192.168.2.7/24 brd 192.168.2.255 scope global enp0s3
       valid_lft forever preferred_lft forever
    inet6 fe80::cf19:776a:4d5c:36e0/64 scope link
       valid_lft forever preferred_lft forever

Below is my /etc/network/interfaces

# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback

Output of nmcli conn sh

Wired connection 1  42d740b1-21c3-3b7c-8b9e-5922fe8a6380  802-3-ethernet  enp0s3

ifup and ifdown are used to set up interfaces listed in /etc/network/interfaces . enp0s3 isn't described in this file, so it's absolutely expected that ifup and ifdown don't know what to do with it. Add some lines such as

iface enp0s3 inet dhcp

And you'll see that it will work much better.

I used the following commands:

cat /proc/net/dev
ls /sys/class/net
ip link show
ifconfig

sudo ifconfig [interface-name] down / up

Network issue. Ifup says unknown interface

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