Can I add a note on the Android app. I am premium.
Yes, you can.
In the app, tap on a task; the fourth icon at the bottom gives access to the note screen, where you can view, add, edit, and delete notes. The icon looks like a small paragraph of text (three lines and a half).
In the widget, tap on a task; the last icon at the top is the usual "More" Android menu (three vertical dots); in that menu, there is a Notes entry that gives access to the note screen.