Tente isto:
sudo rm -r /etc/apt/sources.list.d/enpass.list
sudo apt-get update
sudo apt-get install curl
curl -s http://repo.sinew.in/keys/enpass-linux.key | sudo apt-key add
echo "deb http://repo.sinew.in/ stable main" | sudo tee /etc/apt/sources.list.d/enpass.list
sudo apt-get update
sudo apt-get install enpass