Not able to get sudo update on Ubuntu - ubuntu-18.04

Question

I have an issue with my 18.04 Ubuntu Server.
When I try to run "sudo apt-get update" I get this (code 1)
Also I get this error (code 2) when I try to start some services for mastodon (joinmastodon.org)
CODE 1
root#dedi-par-72088:~# sudo apt-get update
Hit:1 http://mirrors.online.net/ubuntu bionic InRelease
Hit:2 http://mirrors.online.net/ubuntu bionic-updates InRelease
Hit:3 http://mirrors.online.net/ubuntu bionic-backports InRelease
Hit:4 http://archive.ubuntu.com/ubuntu bionic InRelease
Hit:5 http://ppa.launchpad.net/certbot/certbot/ubuntu bionic InRelease
Hit:6 http://security.ubuntu.com/ubuntu bionic-security InRelease
Hit:7 https://deb.nodesource.com/node_8.x bionic InRelease
Get:8 https://dl.yarnpkg.com/debian stable InRelease [13.3 kB]
Err:8 https://dl.yarnpkg.com/debian stable InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 23E7166788B63E1E
Reading package lists... Done
W: An error occurred during the signature verification. The repository is not updated and the previous index files will be used. GPG error: https://dl.yarnpkg.com/debian stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 23E7166788B63E1E
W: Failed to fetch https://dl.yarnpkg.com/debian/dists/stable/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 23E7166788B63E1E
W: Some index files failed to download. They have been ignored, or old ones used instead.
CODE 2
Failed to start mastodon-web.service: The name org.freedesktop.PolicyKit1 was not provided by any .service files
See system logs and 'systemctl status mastodon-web.service' for details.
Failed to start mastodon-sidekiq.service: The name org.freedesktop.PolicyKit1 was not provided by any .service files
See system logs and 'systemctl status mastodon-sidekiq.service' for details.
Failed to start mastodon-streaming.service: The name org.freedesktop.PolicyKit1 was not provided by any .service files
See system logs and 'systemctl status mastodon-streaming.service' for details
CODE 2.1
mastodon#dedi-par-72088:~$ systemctl status mastodon-streaming.service
● mastodon-streaming.service - mastodon-streaming
Loaded: loaded (/etc/systemd/system/mastodon-streaming.service; disabled; vendor preset: enabl
Active: inactive (dead)
CODE 2.2
mastodon#dedi-par-72088:~$ systemctl enable mastodon-*
Failed to enable unit: The name org.freedesktop.PolicyKit1 was not provided by any .service files
I think this is a Networking issue, but I have no idea how to fix it.
Thank you in advance.

Answer 1

To add the latest key for yarn run
curl -sS https://dl.yarnpkg.com/debian/pubkey.gpg | sudo apt-key add -

Answer 2

the issue of "apt-get update" in CODE 1 is that you are missung the public key 23E7166788B63E1E.
You can add this key to your keyring through executing:
sudo apt-key adv --recv-key 23E7166788B63E1E
It will show you something like:
Executing: /tmp/apt-key-gpghome.6KxIWRrerk/gpg.1.sh --recv-key 23E7166788B63E1E
gpg: key 1646B01B86E50310: public key "Yarn Packaging <yarn#dan.cx>" imported
gpg: Total number processed: 1
gpg: imported: 1
Now you can run "apt-get update", or "apt update" and it will fetch the index of https://dl.yarnpkg.com/debian/dists/stable/InRelease as well.

Answer 3

For this one
sudo apt-get update
I think I know the answer. First identify the Apps that make problems. Here it seems yarn. Then GO TO /var/etc/apt/sources.list.d and delete the related 2 files to that App in this folder.
Worked well for mine.