What’s happening here is that you only have a system-wide remote configured, but you’re trying to install as user. Flatpak keeps two separate sets of remotes:
System remotes (available to all users)
User remotes (only for the logged-in user)
Since you have no user remotes defined, Flatpak can’t find anything when you do --user.
You need to add Flathub as a user remote. Run:
flatpak remote-add --if-not-exists --user flathub https://flathub.org/repo/flathub.flatpakrepo