Export Monthly Landsat NDVI Data from GEE

Question

I am trying to export monthly MODIS Terra Daily NDVI data from Google Earth Engine (GEE). Despite researching various platforms, including this one, I find myself at an impasse. I attempted to implement the solution shared by Ludmila as it closely aligns with my requirements. However, I encountered the following error:

Image (Error) Image.reduce: Unable to reduce an image with 0 bands.

I have novice understanding of GEE. Below is the code I'm using:

// Set years and month
var startYear = 2000;
var endYear = 2023;
var years = ee.List.sequence(startYear, endYear);
var months = ee.List.sequence(1, 12);

// Load the image collection
var dataset = ee.ImageCollection('MODIS/MOD09GA_006_NDVI')
                  .filterDate('2000-01-01', '2023-02-01')
                  .filterBounds(ACC);

// Make monthly summed mosaics
// Loop over the years and months to get summed monthly images
var byMonth = ee.ImageCollection.fromImages(
  years.map(function(y) {
    var yearCollection = dataset.filter(ee.Filter.calendarRange(y, y, 'year'));
    return months.map(function(m) {
      var summedImage = yearCollection.filter(ee.Filter.calendarRange(m, m, 'month'))
        .reduce(ee.Reducer.sum());
      var date = ee.Date.fromYMD(y, m, 1);
      return summedImage.set('system:time_start', date)
        .rename(date.format("MM_dd_YYYY"))
        .divide(10)  // Divide by 10 for scaling, if necessary
        .toFloat();
    });
  }).flatten());

// Filter the empty ones out
var outputMonthly = byMonth.filter(ee.Filter.listContains('system:band_names', 'constant').not())
  .sort('system:time_start').toBands();

// Print the resulting mosaic
print("Output Monthly NDVI Mosaic:", outputMonthly);

// Export the data
Export.image.toDrive({
  image: outputMonthly,
  description: "NDVI_ACC_2000-2023",
  scale: 3000,
  folder: "NDVI_ACC",
  region: ACC,
  maxPixels: 1e13  // Adjust this as needed
});