Most commonly, this error means that you are trying to do something to an actor that no longer exists.
For example, if you have a behavior or event that kills the actor, and then a moment later, if you try to move, or obtain a value from that actor (or kill it again), the 1009 error will occur.
One way to avoid the error is to enclose the action that is attempting to access the actor in a [if actor exists] block.