Dear Nitin,
Please test leaving T012K-BNKN2 empty, the iban is entered in FI12
clicking IBAN, which will be stored in TIBAN based on the main bank
account.
If T012K-BNKN2 is maintained, this is the value that will be checked
not IBAN and in the file IBAN is specified not alternative account
number which cannot be definitively longer than 24 characters.
Regards
Gaurav