B2B sales in Aug 2019 but not filed GST 1 and 3B

A client made B2B sales in August 2019 but failed to file the relevant GST returns, despite the customer claiming Input Tax Credit (ITC). Nil returns were filed from August 2019 to December 2019 in January 2020. The user is seeking advice on how to rectify this error, specifically asking about potential penalties and whether the missed invoices need to be declared in the next GSTR 1 and GSTR 3B returns, with a suggestion to include interest.