Interface not configured - I can't use ifup or ifdown

I need a simple way to bring lan and wireless devices up and down in a script. Hence ifupdown. I have configured /etc/network/interfaces and NetworkManager.conf as shown below. What am I missing?

uname -a
Linux Bill-Ubuntu 4.15.0-118-generic #119~16.04.1-Ubuntu SMP Tue Sep 8 14:45:47 UTC 2020 i686
i686 i686 GNU/Linux

cat /etc/network/interfaces
# interfaces(5) file used by ifup(8) and ifdown(8)
auto lo
iface lo inet loopback
iface enp8s8 inet static
address 192.168.1.165
netmask 255.255.255.0

cat /etc/NetworkManager/NetworkManager.conf
[ifupdown]
managed=true

ifconfig
enp8s8 Link encap:Ethernet HWaddr 00:16:d4:32:8c:3d
inet addr:192.168.1.165 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::b521:b762:ca4a:d0c6/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:6459113 errors:0 dropped:0 overruns:0 frame:0
TX packets:1234859 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1557602946 (1.5 GB) TX bytes:137259083 (137.2 MB)

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:20152 errors:0 dropped:0 overruns:0 frame:0
TX packets:20152 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1842616 (1.8 MB) TX bytes:1842616 (1.8 MB)

wlan Link encap:Ethernet HWaddr 7c:a7:b0:be:e5:8f
inet addr:192.168.1.175 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: fe80::23d4:aee5:eb73:11a9/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:8524 errors:0 dropped:126945 overruns:0 frame:0
TX packets:56 errors:0 dropped:5 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:1589094017 (1.5 GB) TX bytes:1612227 (1.6 MB)

sudo ifdown enp8s8
ifdown: interface enp8s8 not configured