Booking.com provides the option to set up multiple rate plans for the same listing.
For example, set one price for rooms with free cancellation and another for rooms with breakfast included.
From Hostify you can control only one Booking.com rate, which usually is the Standard one.
New rates must be added from Booking.com Extranet portal -> Rates & Availability -> Rate Plans and must be set to be based on/Linked the Standard one which is controlled from Hostify.
Also, newly added rate plans must be XML active.
To change a rate plan to XML active, submit a request via your Booking.com account or call directly your Booking.com account manager and ask him to do it for you.