Empty request body in server, Object object into DB

I'm using NodeJS+express and React. The request body's expected output is "tipologia", but it actually returns an empty object. I have looked for similar questions (there a lot of them) but none of these is useful.

client:

function CreateStudyPlan(tipologia){
  return new Promise((resolve, reject) => {
    fetch((URL+'/PianoStudio'), {
      method: 'POST',
      credentials: 'include',
      headers: {
            "Access-Control-Allow-Headers" : "Content-Type",
            "Access-Control-Allow-Origin": "*"
      },
      body: JSON.stringify(tipologia),
    }).then((response) => {
      if (response.ok) {
        resolve(null);
      } else {
        // analyze the cause of error
        response.json()
          .then((message) => { reject(message); }) // error message in the response body
          .catch(() => { reject({ error: "Cannot parse server response." }) }); // something else
      }
    }).catch(() => { reject({ error: "Cannot communicate with the server." }) }); // connection errors
  });
}

server:

// set-up the middlewares
    app.use(morgan('dev'));
    app.use(express.json());
    const corsOptions = {
      origin: 'http://localhost:3000',
      credentials: true,
    };
    app.use(cors(corsOptions));

const isLoggedIn = (req, res, next) => {
  if(req.isAuthenticated())
    return next();
  
  return res.status(401).json({ error: 'not authenticated'});
}

app.post('/PianoStudio', isLoggedIn, async (req, res) => {
  try {
    await dao.createPianoStudio(req.body, req.user.id);   
    res.status(201).json(req.body);
  } catch(err) {
    console.log(err);
    res.status(503).json({error: `Database error during the creation of piano di studi for user ${req.user.id}.`});
  }
});

The problem is that req.body is empty and should not be ( i am expecting it to output part-time):

The insert into the DB shows that req.user.id is ok, while req.body is an empty Object:

--

2 WORDS ON REQUEST ID AND BODY: req.body should be the

body: JSON:Stringify(tipologia)

from the client, while req.user.id is retrieved by the session through the isLoggedIn.

2 WORDS ON HEADERS:

At first i had

headers: {
    'Content-Type': 'application/json',

But it gave me CORS error:

Access to fetch at 'http://localhost:3001/PianoStudio' from origin 'http://localhost:3000' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

So i changed the Headers to

headers: {
            "Access-Control-Allow-Headers" : "Content-Type",
            "Access-Control-Allow-Origin": "*"
      }

as putting 'Content-Type': 'application/json', returns again CORS error.