Are you working on a phone or on a computer?
I'm also using a PRO account; subtasks ARE visible on my Android app.
On the opposite, on the web version (or on the Mac client) subtasks option becomes visible AFTER the task was created. In other words, we have to create the task first, then re-open it in order to add subtask(s).
I must admit it could be more straightforward...
My account is Pro status but nothing pro is working