Thanks – this is a known issue which we expect to be resolved with the release of Endless OS 3.7.0, currently due in late October. Update: we plan to make a 3.6.4 release in the next few days. In the meantime you can work around this issue by installing these apps from the command line:
flatpak install flathub com.valvesoftware.Steamflatpak install flathub com.spotify.Clientflatpak install flathub org.videolan.VLC
You will see a warning, but the installation of the app itself will succeed.