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