Section 73(11) - Goods and Services Tax - GST

FY 2018-19 RCM liability paid at the time of GSTR 9 / 9C on Feb 2021 but ITC not claimed. Now the GST officer is demanding interest U/s 50 and penalty U/s 73(11) . Whether penalty u/s 73(11) is applicable? Any remedy/ argument available to us against penalty. Thanks