Parametric shape items cannot be pasted to masks. Draw the circle directly as a mask.
Ok thanks appreciated.
To elaborate on what Mylenium said: Parametric shapes don't have paths, so there's no path to copy and paste, but you can also make shapes using paths. Just hold the Option or Alt key when creating a shape with a shape tool to create a path-based shape instead of a parametric shape. Then you can copy and paste and link the path between masks, paint strokes, et cetera.
The After Effects CC (12.2) update makes creating Bezier paths easier and more obvious.
option for creating shape layers based on Bezier paths:
When a shape tool (Rectangle, Rounded Rectangle, Polygon, Star, or Ellipse tool) is active, you can use the new Bezier Path option in the Tools panel to create a new shape based on a Bezier path, as opposed to the default of creating a new shape based on a parametric path. Holding the Alt (Windows) or Option (Mac OS) key while drawing a shape causes the opposite behavior—i.e., if the Bezier Path option is enabled, holding the Alt or Option key causes the shape tool to create a parametric path; if the Bezier Path option is disabled, holding the Alt or Option key causes the shape tool to create a Bezier path.
command for converting a parametric shape layer path to a Bezier path:
You can convert a parametric path to a Bezier path after the parametric path has already been created by context-clicking (right-clicking or Control-clicking on Mac OS) the property group for the parametric path (e.g., Rectangle Path 1) and choosing the Convert To Bezier Path command from the context menu. If the parametric path is animated (keyframed), the converted Bezier path is a static path based on the parametric path at the current time; keyframes are lost.
IMPORTANT: When you use the Convert To Bezier Path command to convert a parametric shape path to a Bezier shape path, the Bezier path that is created does not animate well (i.e., interpolation between paths behaves strangely and unpredictably). This is related to path direction and how transformations are stored. For now, you should not use these converted paths for animated paths (interpolation between paths); but, if you do want to try, you may be able to work around the issues by reversing the path before conversion.