When the force and mass of an object are kept constant, there is no change in velocity or speed because of Newton's first law of motion, also known as the law of inertia. This law states that an object at rest will remain at rest, and an object in motion will continue moving with a constant velocity, unless acted upon by an external force.
When the force acting on an object is balanced by an equal and opposite force (resulting in a net force of zero), the object will remain in a state of equilibrium. In this case, there is no unbalanced force to cause a change in velocity or speed.
However, this does not mean that there is no acceleration. Acceleration is defined as a change in velocity over time. Even if the object's velocity remains constant, there can still be acceleration if the object changes its direction of motion. Acceleration is a vector quantity and includes changes in both speed and direction.
So, if an object with constant force and mass changes its direction of motion, there will be acceleration, but the magnitude of the velocity (speed) will remain constant.