Flatpak comes pre-installed in openSUSE Leap 15.2. However, executing the flatpak command for the first time had a minor quirk in a freshly installed Leap 15.2 machine.

error: While opening repository /var/lib/flatpak/repo: opendir(objects): No such file or directory

I see other users commenting about this on Reddit. The error is "most probably" due to missing repositories. Therefore, we will add the Flathub repository as described in its documentation.

flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo

Voilà! Now, you may search apps and install them with the flatpak command. For example, if you would search for Visual Studio Code, you would do:

flatpak search code

Find its Application Id from the result obtained with the above command and install it as follows:

flatpak install flathub com.visualstudio.code

The installation command arguments format is:

flatpak install repo_name application_id

In order to remove an application that you previously installed through Flatpak, just replace install by remove followed by the Application Id and it's done.