Just recently got my new Tab S7 Plus and was wondering why my Samsung Free wasn't showing the Watch tab. I read in the notices that it said it can take a day to show up. I got impatient to fix this so I went in search of a solution, to which I never found.
Anyway long story short, I found the simplest solution was to clear the data in the app and voila it showed up. So anyone who may have this problem (and who cares), try this and see if it works for you.
No it's the Samsung Free app, however Samsung TV Plus is essentially the watch portion of that app. Samsung Free is the homescreen version which includes the news, games and now the free TV part in one app.