How to lay tile backer board on sandy cement floor
You have to get rid of the sandy area/sand.   If there is just a thin layer and you have solid flooring under you can use a filler like a floor leveler - there are other options but I would need to see pictures.   If there is a thick layer or no solid flooring you are going to have to pour concrete or frame (which may be impossible).   

There are solutions like floating a floor over the sand but these would break down over time - probably a lot faster than you think.   This is maybe an alternative in some climates and some situations.