Courtesy intimation about return filing default

My GST Registration was a casual registration which came in effect from 17/01/2018 and was expired on 11/04/2018. I have filed all my returns for the above time period and after that GST portal didn’t allow me file the further returns may be due to my period is expired. But now the Maharashtra Goods and Services Tax Department GST Bhavan, Mazgaon, Mumbai-400010 issuing me Courtesy intimation about return filing default form May 2018 to November 2018, as my registration is already expired in April 2018 how can I file further period returns?, kindly suggest what should I do?