I dont’ understand why I have a message error with my ROUTE POST API?

I am a project, and my goal is to create an API. During authentication a user can create a sauce with different fields. I am working with mongoDB. When I submit the form, the image saves well in my backend, but nothing on the page and nothing on collection (mongodb atlas).

This is what I have to respect :
request body :{ sauce: String,image: File }
type of response :{ message: String }Verb
And which is recorded from the frontend. I just have to take care of the backend and not touch anything on the frontend side.

I have successfully connected in my mongoose apps.js file, and I have no error on the connection. But as the form is not displayed, nothing is sent on mongodb atlas too.

I have this message : SyntaxError: Unexpected token u in JSON at position 0
at JSON.parse ()
at exports.createThing .

exports.createThing = (req, res, next) => {
  const thingObject = JSON.parse(req.body.thing);
  delete thingObject._id;
  const thing = new Thing({
    ...thingObject,
    imageUrl: `${req.protocol}://${req.get("host")}/images/${
      req.file.filename
    }`,
  });
  thing
    .save()
    .then(() => res.status(201).json({ message: "
registered object !" }))
    .catch((error) => res.status(400).json({ error }));
};