If the steel doesn't move then how can I interpretate this ?
The interpretation you mentioned is completely wrong.(why?)
Therefore speed is same then how can the speed of the ball can be halved after the collision. Is this because of inelastic collision?
No!. Consider the steel is initially at rest and the plastic ball is moving towards it. If the collision is taken place only between these two bodies ( i.e system under consideration is isolated) then even if the collision is inelastic the velocity of the steel(was at rest before collision) which is been hit after the collision can not be $0$ even if it were a completely inelastic collision.
The reason why after the collision velocity of steel should not be $0$ is that when the collision takes place both the bodies impart equal and opposite forces on each other.Since a force is acting upon the steel it will increase the velocity from $0$( because initially steel was at rest) to a certain value.
Then how should we interpret the collision.
Let us suppose the system is isolated then the momentum of the system should remain conserved i.e
$$ m_1 u_1 +m_2 u_2 = m_1v_1 +m_2 v_2 $$
$$ \Longrightarrow m_1u_1=m_1v_1 $$ $$\Longrightarrow u_1=v_1 $$
This result is contrary to the observation($ u_1=-1/2 v_1 $). This implies that our assumption that the system is isolated is wrong.
A better interpretation is that the system under consideration contains more than two bodies. For example the steel might be placed right behind another object(of same mass as that of steel) in such a way that when the collision occurs the plastic body impart some force on steel and steel acquires a velocity but suddenly this steel collides with the third body of same mass as that of steel elastically. Now the collision of steel with third body will cause the steel to stop momentarily and the third body will start moving with the speed with which the steel should have been moving. Also since the steel was placed right behind the third body it appears as the steel remains stationary all along.