Can Someone Clarify This To A Beginner

Photoshop UXP Plugin API
5

You’re close to the solution, but itemIndex is a property of the Layer Descriptor and you can’t target specific properties inside a reference. That would be as if you’d say “Select the layer that has opacity 50%”.

You can target a property, but it will be nested inside it’s own object inside the _target array, for example:

{
  _obj: 'get',
  _target: [{ _property: 'itemIndex' }, { _ref: 'layer', _enum: 'ordinal', _value: 'targetEnum' }],
}

(This would get the itemIndex of the selected layer).

However, what you want is to specify the layer reference. For references, there are a handful of keywords that you can use, such as _name, _index or _id. (example)