While the person's speed (magnitude of velocity) may be constant at 7 mph, the direction of their velocity is constantly changing as they move around the circular track. This means that their velocity is not constant, and therefore their acceleration is not zero.
Acceleration is the rate of change of velocity, and since the person's velocity is changing, their acceleration is non-zero. In this case, the acceleration is directed towards the center of the circular track and is called centripetal acceleration.
