Information about the structure of the IBAN is now stored in Microsoft Dynamics 365 for Finance and Operations. That information is automatically loaded when you first use the IBAN on bank accounts. In 8.0.4, it contains the length of the IBAN, the starting positions of the bank account number and the length of the bank account number. In 8.1.1, we added the starting positions of the routing number and the length of the routing number.

Note that the validation is a warning and will not stop you from saving the numbers that you have entered. The account number and routing number will not be validated if their length is zero. 

For more information, following this link: 

https://docs.microsoft.com/en-us/dynamics365/unified-operations/financials/cash-bank-management/iban-validation