EntityDieAfterEventSignal not Available

Attempting to create a Mob Stacking Addon for Bedrock Minecraft as its impossible to host an active server due to the number of mobs spawned in.

Receiving Error:
[Scripting][error]-EntityDieAfterEventSignal is not available in this version of the API.

This is what I am using to lower the number on the mobs stacked from example: 10 to 9, when a mob is killed.
 
How can this be made available? I have APIs enabled and the info for this came from the Microsoft Website