CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource flask-react

I'm getting this message when working on the POST method on my frontend. The weird thing is that for the GET method, I'm receiving all the data without cors issues. Do I have to change something?

Back end

app = Flask(__name__)
CORS(app, supports_credentials=True)
mysql = MySQL(app)

@app.route('/addContact', methods=['POST'])
@cross_origin(supports_credentials=True)
def addContact():
    if request.method == 'POST':
        firstname = request.form.get('firstname')
        lastname = request.form.get('lastname')
        birthday = request.form.get('birthday')
        conn = mysql.connection
        cur = conn.cursor()
        cur.execute(
            "INSERT INTO `farm-users`.users VALUES (DEFAULT, {}, {}, {})".format(firstname, lastname, birthday))
        conn.commit()
        return 'success'

Front end

   const addUser = (e) => {
        e.preventDefault()
        createUser = {"firstname": firstname, "lastname": lastname,"birthday": birthday}
        const requestOptions = {
            method: 'POST',
            headers: {  'Access-Control-Allow-Origin' : '*',
                        'Access-Control-Allow-Methods':'GET,PUT,POST,DELETE,PATCH,OPTIONS',
                        'Content-Type': 'application/json',
                        'Access-Control-Allow-Headers': '*'
                    },
            body: createUser
        };
        const res = fetch('http://127.0.0.1:3000/addContact', requestOptions)
        console.log(res)
    }