Belatedly filed monthly Return GSTR 3B

Respected sir🙏. My client filed Mar-2020 3B return late on Dec-2020. Now a notice has come for this. In the GST notice, Violation of Section 16(4) of the Act and following this Section 39 is mentioned in the notice as ineligible for ITC due to late filing. The officer asks to pay ITC with interest for this. Sir, what should I do now?