That error means there’s no flathub remote in your user installation.
First check what’s there:
flatpak --user remotes
If flathub isn’t listed, add it for the user scope:
flatpak --user remote-add --if-not-exists flathub https://dl.flathub.org/repo/flathub.flatpakrepo
Then retry your install:
flatpak --user install flathub org.freedesktop.Platform/x86_64/25.08
See also: