The IBAN field is delivered as 9 international free text boxes and is optional as standard
If you try to make this mandatory you may experience the following issue -->
IBAN may be different lengths - if you make all 9 boxes mandatory some countries will not pass the validation and if you make only some of the UI boxes mandatory other countries may not pass validation
The best you can do is use NWDI/NWDS as per note 1625973 and flag to end user to enter data here