Currently, there is no possibility of error states on 'Create a new post' , 'Add a response' and 'Add a comment' screen because if user didn't entered anything in text fields, the post button will be disabled un till or unless user entered some texts in fields.
Instead of disabling the buttons the best practice is to enable them and show proper error messages on text fields if user try to submit post buttons without entering any text in fields.
1) Login to edX app.
2) Tap on any course card to access any course.
3) Tap on discussions tab.
4) Tap on 'Create a new post', 'Add a response' or 'Add a comment' and view.