Custom field update from Deluge custom function giving permission error

Custom field update from Deluge custom function giving permission error

Hello,

We are trying to create a custom function which will sum the total quantity of all line items on a Sales Order and output this to a custom field on the Sales Order. We have a custom field called "Sum of Line Items" which is a Number type and is a Default Value of 0 (API Field Name is cf_sum_of_line_items). The custom function is written inside of Deluge with Sales Order as the Module chosen:

salesorderID = salesorder.get("salesorder_id");
organizationID = organization.get("organization_id");
sumOfLineItem = 0;
// Loop through line items and add sum to total
allLineItems = salesorder.getJSON("line_items");
for each  lineItem in allLineItems
{
    lineItemQty = lineItem.get("quantity");
    sumOfLineItem = sumOfLineItem + lineItemQty;
}
info sumOfLineItem;

// Create List for Custom Fields update
CustomFields = List();
CustomField = Map();
CustomField.put("label","Sum of Line Items");
CustomField.put("value",sumOfLineItem);
CustomFields.add(CustomField);
soUpdate = Map();
soUpdate.put("custom_fields",CustomFields);
response = zoho.inventory.updateRecord("SalesOrders",organizationID,salesorderID,soUpdate);
info response;

When we execute this code as an Admin user through the Custom Function script edit screen we receive the following response output:
{"code":57,"message":"You are not authorized to perform this operation"}

The sumOfLineItem reports correctly but it will not update the record's custom field "Sum of Line Items" from 0 to 16 (as an example).

My understanding is that working within Automation > Custom Functions and coding in Deluge does not require an explicit API connection to work on this update.  Is that correct? And is the custom_fields area the correct list to update or is custom_field_hash the one I should be attempting to update? (attempts to update both result in the code 57). What syntax or update method is incorrect in this code?

Thank you,
Curtis