When an object's acceleration is zero, it means that there is no net force acting on the object. In this situation, the object may continue to move with a constant velocity if it was already in motion, or it may remain at rest if it was initially at rest.
The concept you mentioned, that an object has maximum velocity when its acceleration is zero, applies specifically to situations where the object is subject to an opposing force, such as friction or air resistance. These opposing forces act to decrease the object's velocity, and at some point, the force of friction or air resistance may become equal in magnitude and opposite in direction to the applied force, resulting in a net force of zero.
When the net force is zero, the object reaches its maximum velocity because there is no additional force to further accelerate or decelerate it. At this point, the object is said to have reached its terminal velocity. Terminal velocity is the constant velocity attained by an object when the resistance forces acting upon it (such as air resistance or friction) balance out the force causing its motion (such as gravity).
It's important to note that in the absence of any external forces or resistance, an object can continue to accelerate even when its acceleration is not zero. According to Newton's second law of motion, an object's acceleration is directly proportional to the net force acting on it and inversely proportional to its mass. Therefore, if there is a net force acting on an object, its acceleration will not be zero, and it can continue to increase its velocity.