HTTP Code
400

Error Message
Order quantity is greater than Offer quantity
or
Quantity_reserved should be greater than 0 and less or equal to Offer quantity
or
Quantity_reserved should be greater or equal to quantity_in_stock

Possible Cause and Solution
It's not possible to make Order for specified quantity, because your Order quantity is greater than available Offer quantity.
Check that quantity_reserved of your Reservation has correct value. This may occur when you attempt to update your Reservation.