Speed is a scalar and velocity is a vector. Scalars do not depend upon path taken and velocity being a vector does depend on path taken. Therefore if we know a constant velocity we can know the position at a certain time, however if we only know of a constant speed then that doesn’t tell us the direction only the magnitude, not magnitude and direction as would be with velocity. So we would not be able to tell what the position was at a certain time only knowing off a constant speed and not a constant velocity.
Hope this helps. Any questions feel free to ask. Thank you