Brisk menu crash, software repository issues, unmet dependencies

You have to execute the main command again:

echo "deb [arch=amd64] https://updates.signal.org/desktop/apt xenial main" | sudo tee /etc/apt/sources.list.d/signal-xenial.list

It will end with filling the /etc/apt/sources.list.d/signal-xenial.list file with single line:

deb [arch=amd64] https://updates.signal.org/desktop/apt xenial main

And then you need to update package lists and install the Signal package:

sudo apt update
sudo apt install signal-desktop