What is the acceleration of an object with a constant velocity

Zero.

Acceleration is defined as the change in velocity over time.

Since in your case there is no change, there is no acceleration, so it is zero:

Or in formula: a = Δv t

Where a=acceleration, Δv = change in velocity and t=time

Acceleration is 0 m/s/s, Net force is zero. An object moving at constant velocity does not accelerate and does not have an unbalanced force acting on it.