Why can’t I redirect to my failure page using node.js?

I am working on my newsletter project using Mailchimp API, it works but I can’t redirect to a failure page if the status code is not 200, the browser says localhost refused to connect meanwhile success page redirection works great.
here’s my code:

app.post("/", async(req, res, next)=>{
  const {fName, lName, email} = req.body.user;
  const response = await mailchimp.lists.addListMember("List_ID", {
    email_address: email,
    status: "subscribed",
      FNAME: fName,
      LNAME: lName,
  if(res.statusCode === 200){
    res.sendFile(__dirname + "/success.html");
    res.sendFile(__dirname + "/failure.html");