Find Look At (Relative) Rotation returns a rotator.
can you explain more details in this please ?
The script is fine, but calling it once in CS is not enough. Call it during the timeline update.
did that too still not working 
can you show me how you write it?
Can we see it connected to the TL?
You never explained what tracked actor is, and how the light is supposed to know which to track.
i select a cube which the position is at 0,0,0 then the light is moving around the cube using spline.
That event tick is deselecting everything. You don’t need it. You’re actually dereferencing stuff.
Run the script you had in CS during timeline. Instead of getting rotation from the spline, use the rotation to the target actor.
what do you mean run the script during timeline? can you elaborate please? sorry.
When the TL moves an actor along the spline, it updates its position. Since the position changes every frame, the rotation must also be updated every frame.
The script you have in the CS fires only once, or never if the actor is already in the Level Blueprint - it probably is, seeing how you managed to pick a cube as target.
The find look at rotation script needs to be executed in the same node chain as the Timeline Update white execution wire.
You’re getting rotation from the spline, instead use the rotator the script in the CS produces.
You said you did that, too. Can we see it?