Unable to Add Second Guest to International Booking on Booking.com

We made a reservation through Booking.com for an international trip, but we're unable to add a second guest to the room. A month ago, when I booked a different room, the option to add a guest was available, but now it's not, and we can't add the second guest. Since this is an international trip, we need the document to prove the booking for our visa interview. I’ve emailed Booking.com, but I keep receiving copy-paste responses. If anyone knows how to resolve this, your help would be greatly appreciated.