This looks to be an XFCE problem. I’m running Dungeons & Dragons Online installed via Steam and while trying to open the store, I get this from the terminal.
Failed to call portal: GDBus.Error:org.freedesktop.DBus.Error.UnknownMethod: No such interface ?org.freedesktop.portal.OpenURI? on object at path /org/freedesktop/portal/desktop>
This used to work well in the last version of XFCE, I believe I am running 4.16. Not sure what’s changed and how to fix it.