Average speed = (total distance covered) / (time to cover the distance).
Runner's average speed = (length of 1 lap) / (her time to run 1 lap).
________________________________
Average velocity =
(straight-line distance between start-point and end-point)
divided by
(time to go from start-point to end-point).
The runner's start-point and end-point are the same place.
The distance between them (her 'displacement') is zero.
Her average velocity = zero .