You did not adjust the containing box. Povray does not know in what range you
need to render your isosurface, and you left it on default, which is a box from
-1 to 1 in each direction. This is too small for you sphere, so everything is
clipped by this box.
You need to specify for example:
contained_by {box {<-3,-3,-3>,<3,3,3>} }
in your isosurface block.

You should also look at the max_gradient parameter, if it is too low (if your
function has sharp variations), you will see holes/black spots in your surface.
If it's too high, the render will be slow.

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