can't install GIMP Flatpak

I'm reading GIMP's Flatpak documentation here. I go ahead and issue the following command after installing Flatpak on my Ubuntu 18.04 system

flatpak install --user 

and I get the following error

error: The Application requires the which was not found

What went wrong? Why does it tell me I don't have the GNOME platform. I'm using Bionic Beaver which by default uses GNOME.

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