Check your Trainz Update Stream in Launcher. If the Trainz Plus stream is missing then put in a HelpDesk request.
I also started with TRS19 Platinum as my fully purchased copy of Trainz. I then took up the subscription offer (initially monthly then annual) and my TRS19 PE was immediately updated to Trainz Plus - it was the same program but it is your username and registration that determines what you see on the screen and what features you get. When Trainz Plus SP5 was released, Surveyor 2.0 (S20) appeared as as option along with Surveyor Classic (or Surveyor 1.0) in the Tools Menu.
When TRS22 Early Access was released my Trainz Plus became TRS22 (according to the message on the top menu bar of the screen but still shows as Trainz+ on the main graphic in the Launcher window) and I still have all the Trainz+ features.
If you have TRS19 (any version) or TRS22 without a subscription then you will not have S20 as an option. If you have TRS22 installed as well as Trainz Plus then both versions will have S20 available as options.