Unable to configure IBAN

I have an IBAN starting with GB-, I'm unable to set it as both payment methods.

- Receiving payouts from Booking.com

- Direct debit payments to Booking.com

Provided error is : "The IBAN you have provided is not valid. Please try again."

I double check myself the IBAN value and I also checked with a validator website, value itself it's correct.

Is this a known behaviour?