ITC CLAIMED IN WRONG HEADS

Interest is only applicable if wrongly availed ITC has been utilized also as per Rule 88B. 

You have to reverse of CGST and SGST amount equivalent to utilized (CGST and SGST) and pay interest of 24% from the date on which your ICGST & SGST balance in credit ledger came down to utilized amount of (CGST and SGST).

Unfortunately, You are not eligible to avail that IGST ITC for the F.Y. 2017-18 because time barred as per subsection 4 of section 16 CGST Act

For more clarity please read rule sub section (3) of Rule 88B of CGST Rule, 2017