Update2: Problem fixed in latest update
Update: This is a bug I hadn't notice yet.
It occurs with the default custom_outcomes.yml file in Minecraft 1.8-1.12.
This will be solved in the next version.
Way to solve it for the moment:
You are using 1.13+ material IDs in Minecrat 1.8.3.
Instead of "GRASS_BLOCK", use "GRASS" and instead of "LEGACY_WOOL", use "WOOL".
Here you have the Minecraft 1.8-1.12 material ID list:
https://www.minecraftinfo.com/idlist.htm