GST on interest for late payment has to be paid in the month in which it is received. Since you have received interest in the month of June,21, then you need to discharge GST liability on the same in GSTR 3B of June 21, not in Jan 21 (Month of Original Bill). Hence you are not required to pay interest on your GST liability i.e. Rs 1440. In case of further assistance, feel free to contact us on [email protected]