Impossible d'installer npm et nodejs

Bonjour,
Je n'arrive pas à faire fonctionner ce script : https://deb.nodesource.com/setup_6.x
puisque plus tard il me retourne cette erreur :

W: The repository 'http://ppa.launchpad.net/chris-lea/node.js/ubuntu artful Release' does not have a Release file.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
W: GPG error: http://ppa.launchpad.net/ondrej/php/ubuntu/ artful InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4F4EA0AAE5267A6C
W: The repository 'http://ppa.launchpad.net/ondrej/php/ubuntu artful InRelease' is not signed.
N: Data from such a repository can't be authenticated and is therefore potentially dangerous to use.
N: See apt-secure(8) manpage for repository creation and user configuration details.
E: Failed to fetch http://ppa.launchpad.net/chris-lea/node.js/ubuntu/dists/artful/main/binary-amd64/Packages  404  Not Found
E: Some index files failed to download. They have been ignored, or old ones used instead.

.
Puis après j'ai essayé un

apt-get install -f nodejs

. Ce qui à peut prêt "marché" puisque ça me renvoyait ensuite en réessayant :

nodejs is already the newest version (4.8.2~dfsg-1).

mais en essayant

node -v

ça me renvoie :

bash: node: command not found

.
Merci d'avance !

PS: J'ai suivi ce lien : https://linuxconfig.org/how-to-install-nodejs-on-debian-9-stretch-linux