Esport with data merge multiple times

I'm trying to create multiple pdfs from a data Merge, but i'm facing many problems. Following in the code 

#target indesign
// copy pasted from somewhere
DataMergeOption.prototype.enablePreview = function(bOn) { 
  var ma = app.menuActions.itemByID ( 108035 ), 
  c = ma.checked, 
  e = ma.enabled; 
  if (bOn.constructor !== Boolean) return false;  
  e && ( ( bOn && !c ) || (!bOn && c ) ) && ma.invoke();  
  return e; 
}; 
var doc = app.activeDocument;
var dmOptions = doc.dataMergeOptions
dmOptions.enablePreview ( false ); // to avoid errors on updateDataSource
var dataMerge = doc.dataMergeProperties;
dataMerge.updateDataSource ();

//fields are numbered, not named
var fieldNames = dataMerge.dataMergeFields.everyItem().fieldName;
var fieldNameToI = {}; // name to array number
for (var i = 0; i < fieldNames.length; i++) fieldNameToI[fieldNames[i]] = i;
var nameI = fieldNameToI["targetFileName"]; // index of the field that contains fileName

var preferences  = dataMerge.dataMergePreferences
preferences.recordSelection = RecordSelection.ONE_RECORD;

// try to export first pdf, 6th row in the data csv
preferences.recordNumber = 6;
dataMerge.exportFile ("C:\myFolder\testOut6.pdf")
preferences.recordNumber = 7;
dataMerge.exportFile ("C:\myFolder\testOut7.pdf")

In the code I want to use the value associated with merge field targetFileName (that has index nameI) not testOut6 or testOut7. 

When saving the pdf I have an error but without description (Unable to export PDF file)

I also tried to access directly to the csv file but I don't know how to detect the file data source (but only something to set https://www.indesignjs.de/extendscriptAPI/indesign-latest/index.html#DataMerge.html#d1e418772__d1e419101 )

Thanks