Impossible to enter IBAN code for Serbian vendor

Hi,

I want to enter the IBAN code for a vendor from Serbia (Country code: RS), but I get the message BF00016: generation not possible/

The bank details are:

IBAN: RS35160053020009138705

BIC: DBDBRSBG

Banca Intesa a.d. Beograd

Milentija popovica 7b

11070 Novi Beograd

SERBIEN

What could be the reason?

Thank you for your advice.

Kind regards,

Linda