Gstr-3b for new registration

Yes, but for the month of August........ by september 20th

I am not able to file Nil Return for the Month of August 2017, My GST registration date is of 24th August 2017. The Error that I am getting is "GSTR3B is not filed for previous period", but the problem is, I am getting only August and September months on my dashboard, there are no previous months available on it. I am confused and I request you to help me with this. Thank you!!!