Constant velocity implies zero acceleration

Acceleration is a vector defined as the rate of change in velocity with time. You can think of the acceleration vector as something that changes the velocity vector. Acceleration can change the velocity's magnitude, its direction, or both.

In this case, the body is moving in a straight line, so the direction of its velocity is not changing, and it is moving with constant speed, so the magnitude of its velocity is not changing. Since neither the direction nor magnitude of the velocity are changing, the acceleration of the body is zero.

In the animation below, you can see that the velocity vector remains constant.