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!

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!