STLCheck is detecting "open edges" - edges which have a poly on one side but not the other. For an object to pass the STLCheck it has to be a continuous surface i.e. a solid object.
Your object is hollow - a single outer surface but no inner surface - try applying a Shell Modifier with a small Inner or Outer Amount as appropriate.
For that kind of object I would question why you're starting with a circle - try using a Line (Editable Spline) to describe the vertical Cross Section of the object (half of it) then apply a Lathe Modifier. Will still require a Shell to pass the STLCheck, but it's a much easier and more flexible way to create such objects.
Max 2016 (SP1/EXT1)
Win7Pro x64 (SP1). i5-3570K @ 4.4GHz, 8Gb Ram, DX11.
nVidia GTX760 (2GB) (Driver 430.86).