Perhaps as the message says, Nationality info for one of your travelling party needs to be updated in the booking, because redemption booking requires input of nationality information. Suspect one of your travelling party is missing this nationality info in their profile / booking, so you can try to do what the message says. Go to manage booking > edit pax details and ensure nationality information is entered and saved before performing the change?

I noticed that nationality info is required for redemption booking, but not for commercial ticket booking.