Unfortunately Wickr was only meant to be installed on Ubuntu 16.04, but because it's linux we can shoehorn some repositories in and call it a day!
From your Ubuntu 18.04 machine, run the following:
echo "deb http://in.archive.ubuntu.com/ubuntu/ xenial main restricted universe" | sudo tee -a /etc/apt/sources.list
sudo apt update
sudo apt install libhunspell-1.3-0 libprotobuf9v5 libpng12-0
From here you can download and install the Wickr clients: