The Rapid Response® system should be laid on as flat a surface as possible. The polystyrene boards are fairly forgiving, but if there are any dips, holes, or undulations in the floor, we recommend using a levelling screed to deal with them. The floor should be dry and free of dust, oil, or other contaminants to ensure that the adhesive sticks properly.
For more information about floor preparation, check out this blog post: How To Install Underfloor Heating On Concrete Floor – Wunda Group.