Flatpak error: REMOTE and REF must be specified

Case: Flatpak 1.8.3 running on Solus Linux 4.1 throws an error when installing a package. Error message: error: REMOTE and REF must be specified

Solution

When installing a pak, for example:

flatpak install org.videolan.VLC

specify the remote source:

flatpak install flathub org.videolan.VLC

Sources

• ubuntu 18.04 – How do I fix flatpak remote errors? – Stack Overflow