Axios
To add a repository for flatpak you have to run it with sudo on Solus:
sudo flatpak remote-add --if-not-exists flathub https://flathub.org/repo/flathub.flatpakrepo
This was already mentioned in some other post and is most likely a bug / misconfiguration in Solus.
You can and should run all other flaptpak commands WITHOUT sudo tho.