Gst annual return - button is not enabled

I am filling nill return, for the year 2018-19 , filled nil return return for GSTR-1 and GSTR3B,  now am trying file nil annual return

While filling annual return , GST3B is prepare online button is not enabled.

What is  the reason ? why prepare online button is disabled model

how i can file annual return for the year 2018-19?

please find the screen shot online.