Apt Update Gives Error for Wireguard

The mentioned PPA owner does not provide packages anymore.

You have to remove it by

sudo apt-add-repository -r ppa:wireguard/wireguard

Instead you have to add universe pocket and install the package from it:

sudo apt-add-repository universe
sudo apt-get update
sudo apt-get install wireguard

In the link https://launchpad.net/~wireguard it says:

This formerly was responsible for producing a PPA for WireGuard on Ubuntu. That functionality has now been folded into Ubuntu itself, so our old PPA has been removed. Simply run apt install wireguard on all Ubuntus ≥ 16.04.

Each apt-get update command is trying to fetch the package information but fails because it has been removed. After removing the ppa from your system using the following command the error should be gone:

sudo add-apt-repository --remove ppa:wireguard/wireguard

Also, it is generally recommended to avoid using PPAs if the package is available on the official archive mirrors of ubuntu unless you need a different version.