Problems with Exercise Tracker

hey there !!
i’m trying for hours to pass the Exercise Tracker project,
i did pass all the demands of the project, all but one,
(the one where u need to add an exercise.)
and i really don’t know where is the problem.
here is my code:

app.post('/api/exercise/add', function(req, res, next){
  let {userId, description, duration, date} = req.body;
  userModel.findById(userId, function(err, doc){
    if(err){ return next(err); }
    if(date){
      date = new Date(date);
    }else{
      date = new Date();
    }
    let myExercise = new exerciseModel({
      '_id': shortid.generate(),
      'userName': doc.userName,
      description, duration, date
    })
    myExercise.save(function(error, data){
      error ?  res.send(error) : res.json(data);
    })
  })
})

thanks a lot for reading !!


Link to the challenge:
https://www.freecodecamp.org/learn/apis-and-microservices/apis-and-microservices-projects/exercise-trackerLink to project