How avoid CORS error between GitHub Pages and Pythonanywhere?

Question

On Pythonanywhere I created a Flask app, to use it as a RESTFul API. My frontend code is hosted on GitHub Pages and it always says that the answer has a CORS Error. From other source I can easily use it without any error. How is it possible?

Here is my python code:

from flask import Flask, jsonify
from flask_cors import CORS

app = Flask(__name__)
CORS(app, resources={r"/api/*": {"origins": "https://username.github.io"}})

@app.route('/api/datagetter')
def get_data():
    my_list = [
        {
            'id': 1,
            'userid': 0,
            'amount': 3
        }
    ]

    return jsonify({'list': my_list})

if __name__ == '__main__':
    app.run(debug=True)

And this is the JavaScript:

function fetchData() {
    fetch('http://username.pythonanywhere.com/api/datagetter')
        .then(response => response.json())
        .then(data => {
            console.log('Fetched data:', data);
        })
        .catch(error => {
            console.error('Error fetching data:', error);
        });
}

As I mentioned if I call the url from the browser, I get the answer, the error is only comes up from the GitHub Pages website. (http://username.pythonanywhere.com/api/datagetter)