creating support for a surface
You generated a simple surface. You do however need a closed body to print. You might get away with using that surface as a cutter for a block and removing the top half, but there is an easier way using blender:


Select the whole ara with A
Extrude with E then Z to constrain direction
pull until you have an item that is fully thick everywhere
Scale the currently selected, extruded vertices with S then Z then 0 to force them all into the same plane
possibly move the vertices down till they all are below the bottom surface


Now you have a positive thickness, solid body! Run a simple "remove double vertices" on this bottom if you want to reduce file size, but there you go! Export as STL and print!