Does a car moving in a circle with constant speed has acceleration or not? Why?

As speed is a scalar quantity, direction of movement doesn’t affect it. But acceleration is a vector quantity as it is the rate of change of velocity which is itself a vector quantity.

So, take uniform circular motion as example,

It’s speed is constant but it has varying velocity as it changes it’s direction so there is always a acceleration acting towards centre which is provided by centripetal force.