Difficulty with importing Vrbo to Booking.com

I have just set up booking.com, I use another 2 platforms to advertise my property. Airbnb and Vrbo... I am able to export booking.com to both airbnb and vrbo BUT Booking.com will not allow me to import Vrbo to its calendar leaving me exposed to having a double booking. I need urgent help. I have followed all the instructions to import Vrbo but Booking.com is saying " this is not a valid iCal URL. Please try again. " I have tried numerous times with no luck. If I can't get it to sync I need to take booking.com off being active. please what do I do