I'm doing a project for my calculus class where we are determining the volume and surface area of a torus.The question I am on is asking me to find the allowable values of "R' and 'r' in the equation y=Sqrt[(r^2 - (x - R)^2)]. It gives us the manipulate shown below, but it does not work. I can manipulate "R" just fine, but when I try to move "r," the picture disappears. Could somebody tell me if this is typed in wrong? Or is there another way to solve this problem?

What I typed in:

Manipulate[
 ParametricPlot3D[
  {(R + r Cos[\[Alpha]]) Cos[\[Theta]], (R + 
      r Cos[\[Alpha]]) Sin[\[Theta]], r Sin[\[Alpha]]}, {\[Alpha], 0, 
   2 \[Pi]}, {\[Theta], 0, 2 \[Pi]},
  PlotPoints -> 50, ImageSize -> Small, Mesh -> None,
  {Axes -> False, Boxed -> False}, PlotStyle -> Brown, 
  ImageSize -> 200],
 {R, 0.2, 0.629961}, {r, 0.2, radiusr}]

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