LMDE6 - No release file

Quick answer

The quick answer to this is, "Have you checked the name for your release is correct?". For LMDE6 try changing faye to bookworm in the sources file.

Reason

The slightly longer answer is that LMDE6 is also the release called Faye so when you use something like,

lsb_release -cs

as in Mullvads install commands for their VPN...

echo "deb [signed-by=/usr/share/keyrings/mullvad-keyring.asc arch=$( dpkg --print-architecture )] https://repository.mullvad.net/deb/stable $(lsb_release -cs) main" | sudo tee /etc/apt/sources.list.d/mullvad.list

Then the line that gets tee'd to your mullvad.list file is,

deb [signed-by=/usr/share/keyrings/mullvad-keyring.asc arch=amd64] https://repository.mullvad.net/deb/stable faye main

Which is correct but as Faye is not an incredibly popular distribution when you go to update apt it gives the error,

E: The repository 'https://repository.mullvad.net/deb/stable faye Release' does not have a Release file.

Which is also correct and fortunately easy to fix by changing the release faye to the release bookworm which is what faye is closely based upon. Problem solved.