Getting errors after a sudo apt update / apt upgrade

[StealthShadow]

Please follow the below template, it will help us to help you!

Expected Behaviour:a

When running:
sudo apt update
sudo apt list --upgradable
sudo apt upgrade

No error should occur after.

Actual Behaviour:

I ran following commands:
sudo apt update
sudo apt list --upgradable
sudo apt upgrade

Following error occurred:
pi@Pihole:~ $ sudo apt update
sudo: unable to resolve host Pihole.lan
Get:1 http://giteduberger.fr rpimonitor/ InRelease [1,933 B]
Hit:2 Index of /debian stretch InRelease
Hit:3 Index of /raspbian stretch InRelease
Err:1 http://giteduberger.fr rpimonitor/ InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E4E362DE2C0D3C0F
Reading package lists... Done
W: GPG error: http://giteduberger.fr rpimonitor/ InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY E4E362DE2C0D3C0F
E: The repository 'http://giteduberger.fr rpimonitor/ InRelease' is not signed.
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.

Also Ran
pihole -r
Completed fine i think.
[i] The install log is located at: /etc/pihole/install.log
Update Complete!

Current Pi-hole version is v4.3
Current AdminLTE version is v4.3
Current FTL version is v4.3.1

Debug Token:

Your debug token is: https://tricorder.pi-hole.net/rypb6oi0se

[DanSchaper]

This is a system level error and not one generated by Pi-hole. It looks like you need to contact rpimontior to see how to get their GPG signing key for their repository.

[StealthShadow]

Hi Dan,
I rather just remove the rpimonitor, can't i do that?

Is this error " sudo: unable to resolve host Pihole.lan " connected to the rpimonitor also?

[DanSchaper]

You could remove the rpimonitor. It's not our project or related to what we do so you'll need to check with them to see how to do it.

[StealthShadow]

Hi Dan, thank you the reply.
I did as you said and contacted him, so far no response.
But i went here:
etc/apt/sources.list.d/rpimonitor.list
And removed that list.

Now i get following.

And why am i getting this error?
unable to resolve host Pihole.lan

[StealthShadow]

So i solved my problem.
And i wanted to share what i did in case someone else has same or similar problem.

It turns out that if the hostname & the hosts don't match it gives this error when you type a sudo command.

So go to /etc/hostname & /etc/hosts, and make sure they match, save the files and your done.
No restart / reboot / reload necessary.