Updating from such a repository can't be done securely (Raspberry pi)

Question

I'm running a 3B+ with Raspbian and I have a problem updating my raspberry pi 3b+

pi@server:~ $ cat /etc/os-release
PRETTY_NAME="Raspbian GNU/Linux 10 (buster)"
NAME="Raspbian GNU/Linux"
VERSION_ID="10"
VERSION="10 (buster)"
VERSION_CODENAME=buster
ID=raspbian
ID_LIKE=debian
HOME_URL="http://www.raspbian.org/"
SUPPORT_URL="http://www.raspbian.org/RaspbianForums"
BUG_REPORT_URL="http://www.raspbian.org/RaspbianBugs"

This is my source.list (i couldn't post it here because it says it's spam) https://pastebin.com/3HCyuf0X and this is the output when I try to run sudo apt-get update https://pastebin.com/VuDM52JA

I have tried to restart and this

sudo apt clean
cd /var/lib/apt 
sudo mv lists lists.old
sudo mkdir -p lists/partial
sudo apt update 

Answer 1

The error message indicates that the public signature key with id 40976EAF437D05B5 of a repository is missed. I don't know where to get the public key but because an Ubuntu repository is missing it, it is possible to get the key from the Ubuntu keyserver. Try to get and import it with:

rpi ~$ sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 40976EAF437D05B5