Verify you don't have a pre-existing manual installation of the mod (a.If the mod background stays red in the launcher after the download has completed in Steam client, it likely means the installation failed: If not, you can force a re-download of the mod by looking into C:\Program Files (x86)\Steam\userdata\\ugc for files & folders referencing the mod (for instance 203770_subscriptions.vdf and \referenced\45371260977302539\mod\), deleting them and restarting Steam client. Verify mod file and archive are present in folder ~\Documents\Paradox Interactive\Crusader Kings II\mod.Note that Steam cloud sync appears to be broken for mods with capitals in user_dir. Make sure you have Steam Cloud enabled.
Important: Remove any previous manual installation of the mod if any.