This error seems to be related to the FINISH EVENT callback.
It appears to be reporting that "_event" is not defined, when it should be. This variable is a list with data about the FINISH callback. _event[| 0] holds a true/false value depending on if the event is enabled/disabled.
It is hard for me to know for sure why this is happening. Which version of TweenGMS are you using? (v1.0.6?)
Shoot me a private DM on Twitter @8BitWarrior (DMs are open) and I can give you my email address for responding with more details.