Also, make sure the booking is still valid, you can't send messages on cancelled bookings. If all else fails, call Booking.com so they can check if the problem lies on their side:
I am on Airbnb (only good for high season) and have been with Booking.Com for 6 months. I was getting up to 3 - 4 bookings a day but that is now majorly
Hiya,
Welcome to the Forum.
What error does it give though?
Also, make sure the booking is still valid, you can't send messages on cancelled bookings. If all else fails, call Booking.com so they can check if the problem lies on their side:
How do I contact Booking.com?
Best of luck