Suppose I am in a spaceship. In this spaceship there is also my friend. So my friend and my relative velocity is zero. So no matter how fast my ship goes my friend's velocity relative to me is zero($0$). So in this logic my ship can go as fast as I like (even greater than the velocity of light).
But a viewer from outside the ship will see the ship going in a velocity. So we won't be able to go in a velocity greater than light. This is of course wrong. Where am I wrong?