XMLHttpRequest发送请求报错：Failed to execute 'send' on 'XMLHttpRequest': Failed to load

const xhr = new XMLHttpRequest();
    xhr.open('post', url, false);
    xhr.setRequestHeader("Content-type","application/json");
    var str = JSON.stringify(uid)
    xhr.send(str);
    const res = JSON.parse(xhr.responseText);
    console.info(res);
   
    return res;