In Roon you can not add tracks to a Tidal playlist. What you can do is: go to the Tidal playlist, hit the three dots, choose ‘Save Local Copy’ (which gets the same name as the Tidal playlist, but the Tidal logo is gone now). Then you can add tracks to the local copy of the playlist. You can also change the name of the new playlist (go to the new playlist > three dots > Edit).
The fact that you can’t add tracks to a Tidal playlist is seen be me - and many in this community as far as I can judge - as a big disadvantage. So far I have seen no news from Roon that this will be addressed anytime soon.