# if your error message is something like:
"E: The repository ‘http://ppa.launchpad.net/$xx/$yy/ubuntu focal Re..."
# then just run this ↓, substituting the $xx and $yy ↑ with what's in your error
sudo add-apt-repository --remove ppa:$xx/$yy
# example, if your error is:
"E: The repository ‘http://ppa.launchpad.net/ubuntudde-dev/stable/ubuntu foc..."
# execute this:
sudo add-apt-repository --remove ppa:ubuntudde-dev/stable

sudo sed -i -re 's/([a-z]{2}\.)?archive.ubuntu.com|security.ubuntu.com/old-releases.ubuntu.com/g' /etc/apt/sources.list