Cannot modify a value type return value of `UnityEngine.Transform.rotation'. Consider storing the value in a temporary variable

I’m taking this error when I use this code;

	SelectedWaypointTransform.rotation.y = transform.rotation.y;

I think I’m missing a small thing but I can’t get that. what is the wrong? Thanks. ._.

SelectedWaypointTransform.rotation = new Vector3( SelectedWaypointTransform.rotation.x,