apt-get update returns error 'failure resolving repo.percona.com'

after trying to get all the updates loaded onto my droplet, update stopped working:

history
 1976  sudo apt-get update
 1977  sudo apt-get upgrade
 1978  sudo dpkg --configure -a
 1979  sudo apt-get upgrade
 1980  sudo apt-get update
 1981  apt-get -f install'
 1982  apt-get -f install
 1983  sudo dpkg --configure -a
 1984  sudo apt-get -f install
 1985  sudo apt-get update
 1986  sudo apt-get upgrade
 1987  sudo apt-get update
 1988  sudo apt-get upgrade
 1989  sudo apt autoremove
 1990  sudo reboot
 1991  sudo apt-get update

The last update returns:

Err:1 http://repo.percona.com/percona/apt xenial InRelease
  Temporary failure resolving 'repo.percona.com'
Err:2 http://repo.percona.com/apt xenial InRelease
  Temporary failure resolving 'repo.percona.com'
Err:3 http://mirrors.digitalocean.com/ubuntu xenial InRelease
  Temporary failure resolving 'mirrors.digitalocean.com'
Err:4 http://mirrors.digitalocean.com/ubuntu xenial-updates InRelease
  Temporary failure resolving 'mirrors.digitalocean.com'
Err:5 http://mirrors.digitalocean.com/ubuntu xenial-backports InRelease
  Temporary failure resolving 'mirrors.digitalocean.com'
Err:6 https://repos.sonar.digitalocean.com/apt main InRelease
  Could not resolve host: repos.sonar.digitalocean.com
Err:7 http://ppa.launchpad.net/certbot/certbot/ubuntu xenial InRelease
  Temporary failure resolving 'ppa.launchpad.net'
Err:8 http://security.ubuntu.com/ubuntu xenial-security InRelease
  Temporary failure resolving 'security.ubuntu.com'
Reading package lists... Done
W: Failed to fetch http://mirrors.digitalocean.com/ubuntu/dists/xenial/InRelease  Temporary failure resolving 'mirrors.digitalocean.com'
W: Failed to fetch http://mirrors.digitalocean.com/ubuntu/dists/xenial-updates/InRelease  Temporary failure resolving 'mirrors.digitalocean.com'
W: Failed to fetch http://mirrors.digitalocean.com/ubuntu/dists/xenial-backports/InRelease  Temporary failure resolving 'mirrors.digitalocean.com'
W: Failed to fetch http://security.ubuntu.com/ubuntu/dists/xenial-security/InRelease  Temporary failure resolving 'security.ubuntu.com'
W: Failed to fetch http://ppa.launchpad.net/certbot/certbot/ubuntu/dists/xenial/InRelease  Temporary failure resolving 'ppa.launchpad.net'
W: Failed to fetch https://repos.sonar.digitalocean.com/apt/dists/main/InRelease  Could not resolve host: repos.sonar.digitalocean.com
W: Failed to fetch http://repo.percona.com/percona/apt/dists/xenial/InRelease  Temporary failure resolving 'repo.percona.com'
W: Failed to fetch http://repo.percona.com/apt/dists/xenial/InRelease  Temporary failure resolving 'repo.percona.com'
W: Some index files failed to download. They have been ignored, or old ones used instead.

My /etc/resolv.conf file:

 cat /etc/resolv.conf
# Dynamic resolv.conf(5) file for glibc resolver(3) generated by resolvconf(8)
#     DO NOT EDIT THIS FILE BY HAND -- YOUR CHANGES WILL BE OVERWRITTEN

 sudo service networking restart
Job for networking.service failed because the control process exited with error code. See "systemctl status networking.service" and "journalctl -xe" for details.

systemctl status networking.service
● networking.service - Raise network interfaces
   Loaded: loaded (/lib/systemd/system/networking.service; enabled; vendor preset: enabled)
  Drop-In: /run/systemd/generator/networking.service.d
           └─50-insserv.conf-$network.conf
   Active: failed (Result: exit-code) since Sun 2018-12-30 11:12:55 EST; 3min 43s ago
     Docs: man:interfaces(5)
  Process: 20195 ExecStart=/sbin/ifup -a --read-environment (code=exited, status=1/FAILURE)
  Process: 20191 ExecStartPre=/bin/sh -c [ "$CONFIGURE_INTERFACES" != "no" ] && [ -n "$(ifquery --read-environment --list --exclude=lo)"
 Main PID: 20195 (code=exited, status=1/FAILURE)

Dec 30 11:12:55 kbcmhub ifup[20195]: Failed to bring up lo.
Dec 30 11:12:55 kbcmhub ifup[20195]: RTNETLINK answers: File exists
Dec 30 11:12:55 kbcmhub ifup[20195]: Failed to bring up eth0.
Dec 30 11:12:55 kbcmhub ifup[20195]: /etc/network/if-up.d/sendmail: 44: .: Can't open /usr/share/sendmail/dynamic
Dec 30 11:12:55 kbcmhub ifup[20195]: run-parts: /etc/network/if-up.d/sendmail exited with return code 2
Dec 30 11:12:55 kbcmhub ifup[20195]: /sbin/ifup: post-up script failed.
Dec 30 11:12:55 kbcmhub systemd[1]: networking.service: Main process exited, code=exited, status=1/FAILURE
Dec 30 11:12:55 kbcmhub systemd[1]: Failed to start Raise network interfaces.
Dec 30 11:12:55 kbcmhub systemd[1]: networking.service: Unit entered failed state.
Dec 30 11:12:55 kbcmhub systemd[1]: networking.service: Failed with result 'exit-code'.

Sendmail is not working