I want to test Flatpak on elementary OS. Since it is built on top of Ubuntu 16.04, it should be possible to install flatpack. Like in flatpack installing guide.
sudo apt install flatpak
It doesn't work.
So how to install Flatpak on Loki?
You're in the right track on installation of Flatpak on Elementary as indicated on their guide. You might not be aware of it, but
add-apt-repository is disabled by default on Elementary (to avoid reckless installation of PPAs). Workaround for that situation have been posted multiple times on this site.
Basically, you can enable it via:
sudo apt-get install software-properties-common
After that, you can proceed to the usual Flatpak installation steps.