Revisiting the GitHub Actions Release system (.flatpak + .deb) by Stefterv · Pull Request #1204 · processing/processing4

Stefterv

This is not how --install-deps-from works.

You'll need to

    commandLine(
        "flatpak",
        "remote-add",
        "--user",
        "--if-not-exists",
        "flathub",
        "https://flathub.org/repo/flathub.flatpakrepo"
    )

first and then --install-deps-from=flathub.