Errors during the installation of the flatpak org.freedesktop.Platform when installing GeForce Now on Linux

I do have to ask: How did tried to install Geforce Now? Because the error you posted has to do with the user having not the permission to install Flatpaks to the system installation. But the solution you mentioned just is to install the runtime manually from the terminal. This implies to me: Whatever you used when trying to install Geforce Now lacks the permission with your user to install to the system installation. So, you circumvented the issue, not actually solved it.