Adding to the database shows the error. What should I do?

Access to fetch at 'http:xxx' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: 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.

My function:

addItem = (e) => {
    e.preventDefault();
    const ob = {
        X: 53.0331258,
        Y: 18.7155611,
    }
    fetch("http://xxx", {
            method: "post",
            headers: {
                "Content-Type": "application/json"
            },
            body: JSON.stringify(ob)
        })
        .then(res => res.json())
        .then(res => {
            console.log(res);
        })
}