While connecting TIDAL to Soundiiz, be sure to use the exact same login method that you use on the TIDAL website.
For example, if you have used "Sign in with Google" on TIDAL, be sure to use "Sign in with Google" when connecting TIDAL to Soundiiz. The same goes for login with a username/password, etc.
Because TIDAL possibly created another account (but empty, with no playlists), and that's the one you have connected to Soundiiz.