AdvancementEvent

This event is fired every time the player earns an advancement. This happens after the advancement has already been earned, so it can not be prevented.

The event is not cancelable.

The event does not have a result.

It might be required for you to import the package if you encounter any issues (like casting an Array), so better be safe than sorry and add the import at the very top of the file.

ZenScript
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import crafttweaker.api.event.entity.player.AdvancementEvent;

AdvancementEvent extends PlayerEvent. That means all methods available in PlayerEvent are also available in AdvancementEvent

ZenScript
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// AdvancementEvent.getAdvancement() as Advancement

event.getAdvancement();