I did get to install the Mullvad repo on Mint. The thing is they use the "lsb_release -cs" command which give the codename of Mint and not Ubuntu.
But if you use
awk -F= '$1=="UBUNTU_CODENAME" { print $2 ;}' /etc/os-release
It will give the codename for Ubuntu used by Mint.
Try this from the Mullvad instruction with this change :
# Download the Mullvad signing key
sudo curl -fsSLo /usr/share/keyrings/mullvad-keyring.asc https://repository.mullvad.net/deb/mullvad-keyring.asc
# Add the Mullvad repository server to apt
echo "deb [signed-by=/usr/share/keyrings/mullvad-keyring.asc arch=$( dpkg --print-architecture )] https://repository.mullvad.net/deb/stable $(awk -F= '$1=="UBUNTU_CODENAME" { print $2 ;}' /etc/os-release) main" | sudo tee /etc/apt/sources.list.d/mullvad.list
# Install the package
sudo apt update
sudo apt install mullvad-vpn
Haha, thanks for the kind words but I did forget to add the source. I did not figure this out by myself. The link