I have the same problem:

sudo apt-get upgrade
Reading package lists... Done
Building dependency tree

Reading state information... Done
Calculating upgrade... Done
The following packages will be upgraded:
nplan
1 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 40.4 kB of archives.
After this operation, 7168 B of additional disk space will be used.
Do you want to continue? [Y/n] y
Err:1 http://ams2.mirrors.digitalocean.com/ubuntu zesty-updates/main i386 nplan i386 0.32~17.04.1
404 Not Found
E: Failed to fetch http://ams2.mirrors.digitalocean.com/ubuntu/pool/main/n/nplan/nplan_0.32~17.04.1_i386.deb 404 Not Found
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?

i have the these problem

root@SolGPS:/etc/apt# apt-get update
Hit:1 https://repos.sonar.digitalocean.com/apt main InRelease
Hit:2 http://ppa.launchpad.net/certbot/certbot/ubuntu zesty InRelease
Ign:3 http://nyc2.mirrors.digitalocean.com/ubuntu zesty InRelease
Hit:4 http://old-releases.ubuntu.com/ubuntu zesty-security InRelease
Ign:5 http://nyc2.mirrors.digitalocean.com/ubuntu zesty-updates InRelease
Ign:6 http://nyc2.mirrors.digitalocean.com/ubuntu zesty-backports InRelease
Hit:7 http://ppa.launchpad.net/inkscape.dev/stable/ubuntu zesty InRelease
Err:8 http://nyc2.mirrors.digitalocean.com/ubuntu zesty Release
404 Not Found
Err:9 http://nyc2.mirrors.digitalocean.com/ubuntu zesty-updates Release
404 Not Found
Hit:10 http://ppa.launchpad.net/jtaylor/keepass/ubuntu zesty InRelease
Err:11 http://nyc2.mirrors.digitalocean.com/ubuntu zesty-backports Release
404 Not Found
Hit:12 http://ppa.launchpad.net/kubuntu-ppa/backports/ubuntu zesty InRelease
Hit:13 http://ppa.launchpad.net/libreoffice/ppa/ubuntu zesty InRelease
Hit:14 http://ppa.launchpad.net/nijel/phpmyadmin/ubuntu zesty InRelease
Hit:15 http://ppa.launchpad.net/ondrej/php/ubuntu zesty InRelease
Hit:16 http://ppa.launchpad.net/otto-kesselgulasch/gimp/ubuntu zesty InRelease
Hit:17 http://ppa.launchpad.net/persepolis/ppa/ubuntu zesty InRelease
Hit:18 http://ppa.launchpad.net/ricotz/docky/ubuntu zesty InRelease
Hit:19 http://ppa.launchpad.net/scribus/ppa/ubuntu zesty InRelease
Reading package lists... Done
E: The repository 'http://mirrors.digitalocean.com/ubuntu zesty Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://mirrors.digitalocean.com/ubuntu zesty-updates Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: The repository 'http://mirrors.digitalocean.com/ubuntu zesty-backports Release' does no longer have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.
root@SolGPS:/etc/apt# apt-get upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages were automatically installed and are no longer required:
linux-headers-4.10.0-28 linux-headers-4.10.0-28-generic linux-headers-4.10.0-30 linux-headers-4.10.0-30-generic linux-headers-4.10.0-33 linux-headers-4.10.0-33-generic linux-headers-4.10.0-35 linux-headers-4.10.0-35-generic
linux-headers-4.10.0-37 linux-headers-4.10.0-37-generic linux-headers-4.10.0-38 linux-headers-4.10.0-38-generic linux-image-4.10.0-28-generic linux-image-4.10.0-30-generic linux-image-4.10.0-33-generic linux-image-4.10.0-35-generic
linux-image-4.10.0-37-generic linux-image-4.10.0-38-generic linux-image-4.8.0-32-generic linux-image-extra-4.10.0-28-generic linux-image-extra-4.10.0-30-generic linux-image-extra-4.10.0-33-generic linux-image-extra-4.10.0-35-generic
linux-image-extra-4.10.0-37-generic linux-image-extra-4.10.0-38-generic
Use 'apt autoremove' to remove them.
The following packages will be upgraded:
nplan
1 upgraded, 0 newly installed, 0 to remove and 0 not upgraded.
Need to get 40.1 kB of archives.
After this operation, 3,072 B of additional disk space will be used.
Do you want to continue? [Y/n] y
Err:1 http://nyc2.mirrors.digitalocean.com/ubuntu zesty-updates/main amd64 nplan amd64 0.32~17.04.1
404 Not Found
E: Failed to fetch http://nyc2.mirrors.digitalocean.com/ubuntu/pool/main/n/nplan/nplan_0.32~17.04.1_amd64.deb 404 Not Found
E: Unable to fetch some archives, maybe run apt-get update or try with --fix-missing?