The Flathub repository provides GNOME Platform runtime, which is used by Nuvola to share common dependencies with other Flatpak apps to reduce disk & memory usage. The Nuvola Apps repository contains Nuvola Apps Service and individual Nuvola web apps.
Unfortunately, Flatpak support in elementaryOS is incomplete, so you need to use terminal commands.
flatpak remote-add --if-not-exists --user flathub https://dl.flathub.org/repo/flathub.flatpakrepo flatpak remote-add --if-not-exists --user nuvola https://dl.tiliado.eu/flatpak/nuvola.flatpakrepo flatpak update
It is necessary to restart your computer to activate new flatpak repositories. Unfortunately, just logging out and logging in is not enough.
Nuvola Apps Service is an optional background service that provides individual Nuvola apps with global shared resources such as a global configuration storage, global keyboard shortcuts, a HTTP remote control server, and a command-line controller. While this service is optional, the installation is recommended.
In order to install Nuvola Apps Service, launch AppCenter, search for Nuvola Apps Service and finally click the Install button.
Alternatively, launch Terminal application, type the following command and press the Enter key to install this package.
flatpak install nuvola eu.tiliado.Nuvola