https://xkcd.com/979/

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

Since you didn't post your solution, here it is for others: you need to add Flathub as a user remote too.

flatpak remote-add --user --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo

I could be wrong but no remote refs refers to not finding a repo for what you are trying to install, so either the flathub repo isnt added which it sounds like it is, or the app you want isnt in that repo.

So you may need to add the repo for that app: https://fcitx-im.org/wiki/Install_Fcitx_5