Gfilui

When I try to update from Ubuntu 17.10 to 18.04 I get the following error message:

E:The repository 'http://ppa.launchpad.net/jonathonf/backports/ubuntu artful Release' does not have a Release file.,
W:Updating from such a repository can't be done securely,
and is therefore disabled by default.,
W:See apt-secure(8) manpage for repository creation and user configuration details.,
W:Target Packages (main/binary-amd64/Packages) is configured multiple times in /etc/apt/sources.list:49 and /etc/apt/sources.list.d/vscode.list:3,
W:Target Packages (main/binary-all/Packages) is configured multiple times in /etc/apt/sources.list:49 and /etc/apt/sources.list.d/vscode.list:3,
W:Target Translations (main/i18n/Translation-en) is configured multiple times in /etc/apt/sources.list:49 and /etc/apt/sources.list.d/vscode.list:3,
W:Target DEP-11 (main/dep11/Components-amd64.yml) is configured multiple times in /etc/apt/sources.list:49 and /etc/apt/sources.list.d/vscode.list:3,
W:Target DEP-11 (main/dep11/Components-all.yml) is configured multiple times in /etc/apt/sources.list:49 and /etc/apt/sources.list.d/vscode.list:3,
W:Target DEP-11-icons (main/dep11/icons-64x64.tar) is configured multiple times in /etc/apt/sources.list:49 and /etc/apt/sources.list.d/vscode.list:3

How do I continue upgrading?

The PPA found under http://ppa.launchpad.net/jonathonf/backports/ubuntu is no full Ubuntu repository but a PPA. In this case it does not contain any bionic packages (yet).

You want to use an official repository or mirror which carries all of the packages you need for an upgrade.

Take a look into your /etc/apt/sources.list. Are there the regular repositories linked?

For Ubuntu 17.10 you should have something like

deb http://de.archive.ubuntu.com/ubuntu/ artful main restricted
deb http://de.archive.ubuntu.com/ubuntu/ artful-updates main restricted
deb http://de.archive.ubuntu.com/ubuntu/ artful multiverse
deb http://de.archive.ubuntu.com/ubuntu/ artful-updates multiverse
deb http://de.archive.ubuntu.com/ubuntu/ artful-backports main restricted universe multiverse
...

and the corresponding deb-src entries.

There might be more *.list files in /etc/apt/sources.list.d/ - there you might have to disable the PPA entry manually.

Ho do you try to do the upgrade? The do-release-upgrade would take care of disabling the PPA entries automatically. As the time of writing this, the official release hasn't happen yet. Therefore a do-release-upgrade would not find the currtly beta status release right now. Either wait some time until they released 18.04 or use do-release-upgrade -d and go with the current status of 18.04.