NodeJS (ExpressJS) Unhandled rejection TypeError: callback is not a function

I've been learning to use Angular 6 and NodeJS (with ExpressJS) these past few days. To do this, I decided to create a front-end with Angular 6 (o which theres a form) with the intention of passing information back and forth to my PHPMyAdmin database via the use of an API I created with NodeJS. As you may have seen from the title, I get the following error when I try to submit my forms information:

"Unhandled rejection TypeError: callback is not a function"

The code I used for the post action was taken from another functioning and similar API, the database receives the question_id and user_id but will not register the questions answers (which is the end goal). Without furthur ado, here is the code to get and post:

const config = require('../config/config');
const knex      = config.db.knex;
const async     = require('async');


class TestModel {        
  getQuestions(callback) {
        knex('question')
        .select('id', 'libelle')
    .orderBy('id', 'asc')
        .then((data) => {
            callback(data);
        });
    }


  addResponse(reponse, callback) {
      knex('reponse')
      .insert({
          id_user : 1,
          id_question : reponse.id,
          libelle: reponse.answer,
      }).then((data) => {
          callback(data);
        });
  }        
}


module.exports = new TestModel();

Here is the rest:

app.post('/quiz', function(req, res, next){
    var answers = req.body;

    console.log(answers)

    for(var i = 0; i<answers.length; i++)
        {
            var obj = answers[i];
            Test.addResponse((obj,result) => {

            });                
        }        
    res.json({reponseserveur:'True'});    
});

Just for reference, "reponse" means response and "libelle" means label. Thanks in advance for your help!