5xx or 4xx error with “No 'Access-Control-Allow-Origin' header is present”

My browser is logging the following message in the devtools console:

No 'Access-Control-Allow-Origin' header is present on the requested resource.… The response had HTTP status code 503.

Background: I have two apps. One that is an Express Node application connected to a Mongo database. The other is a basic web application that makes POST requests to the Node application via the Fetch API to get data from Mongo.

Issue: Though I receive no CORS errors on my local machine, I am given the error below as soon as I deploy my basic web application to production. The web application that makes a POST request to the Node app and gives me this:

The POST request does seem to work and the data is saved into Mongo but this error is being marked as a "Critical Error" in Heroku and is quite annoying.

I realize that I could set the no-cors option in Fetch but I believe that it is required since I am making a request to a url that is different than the origin. Right?

Express Node App Code

In my app.js file I have set the correct headers to ensure that other applications can make requests from different origins

app.js

// Add headers so we can make API requests
app.use(function (req, res, next) {
    res.setHeader('Access-Control-Allow-Origin', '*');
    res.setHeader('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, PATCH, DELETE');
    res.setHeader('Access-Control-Allow-Headers', 'X-Requested-With,content-type');
    res.setHeader('Access-Control-Allow-Credentials', true);
    next();
});

routes/api/api.js

router.post('/users/:url/upload-csv/:csv_name', (req, res) => {
  let csv_name = req.params.csv_name;
  let csv_string = csv_name+req.body.csv_string;

  User.findOne({url: req.params.url})
    .then((user) => {
      if (user.csv_files.length === 0) {
        user.csv_files.push(csv_string);
      } else {
        let foundExistingCSV = false;
        for (var i = 0; i < user.csv_files.length; i++) {
          if (user.csv_files[i].includes(csv_name)) {
            foundExistingCSV  = true;
            user.csv_files[i] = csv_string;
            break;
          }
        }
        if (!foundExistingCSV) user.csv_files.push(csv_string);
      }
      user.markModified('csv_files');
      user.save();

      res.status(204);
    })
    .catch((err) => {
      console.log(err);
      res.status(400);
    });
});

Basic Web App Code

POST request I am making

utils.js

utils.exportToMongo = functions(table, name) {
  var exportPlugin = table.getPlugin('exportFile');
  var csv_string   = exportPlugin.exportAsString('csv');

  // Upload the CSV string and its name to Users DB
  fetch(`${utils.fetchUserURL()}/upload-csv/${name}`, {
    method: 'POST',
    body: JSON.stringify({csv_string: csv_string}),
    headers: new Headers({
      'Content-Type': 'application/json',
      Accept: 'application/json',
    })
  }).then((res) => {
    return {};
  }).catch((error) => {
    console.log(error);
    return {};
  });
}

How can I remove the 503 error? Any insight would be greatly appreciated!