explain why an object that moves round a complete circle will have an angular displacement of 2pi radians but displacement of 0m.

Answer 1

distance is a scalar and the distance is indeed 2 pi R
HOWEVER displacement is a VECTOR and has both magnitude and direction from the starting point to the finish point.
In this case the magnitude of displacement from the start to the finish is ZERO. Your angular direction never changed sign. If you went around the circle then came back, the story would be different :)