Structure App version 5.1 or later is required to run Structure.Gantt.

End of Life:

• For beta and release candidate versions: Beta versions may be scheduled to expire after certain amount of time. In any case, the lifespan of such pre-release version is not supposed to be longer than 3 months. You will need to upgrade to a newer version before end-of-life.
• For other versions: You will be able to use this version of the plugin indefinitely, however, after end-of-life date, the support for the version is limited.