This is a known issue with the current version of the App Center in Endless OS. We expect it to be fixed in a forthcoming update, likely 3.6.1 or 3.6.2. In the meantime the workaround is to open a terminal and run:
flatpak install --system flathub org.freedesktop.Platform.Compat.i386 -y
Please do not report this to the Flathub Steam maintainers.