You MUST create that datum ON THE FLY, so to speak. That is, you MUST create that datum through the axis as a reference for the sketch (right, top, left, etc.) or pattern will NEVER work when trying to pattern a feature around a circle. So, you will pick or create a datum plane (as a sketch plane), then you will pick right, left, top, or bottom, and create a new datum AS THAT REFERENCE PLANE. Then it should work. Good luck!