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:

* 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 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!