The applicable interest and penalty shall apply (i.e. Reason you have not mentioned) on all such reversals which shall be paid through entry in column 9 of Table 6.1 of FORM GSTR-3B.
Due date of payment of tax is 20.08.2018 (Return period July 2018).But this return filed on 20.09.2018.How much interest due.Late fee display autopopulate,but interest not showing in the prescribed table.