I just typed exactly this
ContourPlot3D[x^3+y^2-z^2, {x,y,z} \[Element] Ball[{0,0,0},3]]
and it seemed to work. Notice that \ in front of [Element]. Did you include that? With no space between the \ and the [Element]?
ContourPlot3D[x^3+y^2-z^2, Element[{x,y,z}, Ball[{0,0,0},3]]]
also worked, which is a completely different way of writing this without using \