FeatureSetByRelationshipClass validate error - unknown function

[WilliamDoane9]

I am trying to set up a Field Map which can auto-populate an inspection field in a related table from an existing point

I created the Relationship Class - we can call 'Rel_Name'

Then in Attribute Rules for the table I created a Calculation rule as follows:

"var parent = FeatureSetByRelationshipClass($feature, 'Rel_Name', ['Field_Name'])
return First(parent)"

Validating throws the following error - "Invalid expression. Error on Line 1. Unknown Function."

Can anyone advise?

[KenBuja]

FeatureSetByRelationshipClass was released in the most current version of Arcade (v1.24), which may not work in Field Maps yet. Can you use FeatureSetByRelationshipName instead?