Problem with CORS even after setting header for Access-Control-Allow-Origin

Question

I have an app Vue staying in the Netlify and always when I try to achieve a request of the kind POST in my endpoint https://backend-app.herokuapp.com/clientes I get the following error:

Access to Xmlhttprequest at 'https://backend-app.herokuapp.com/clients' from origin 'http://localhost:8080' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested Resource.

As you can see, I get this error trying to access locally as well.

What I’ve tried in mine server.js:

const cors = require("cors");
app.use(cors());

Still in my server.js added Access-Control-Allow-Origin

app.use( ( req, res, next ) => {    
    res.setHeader("Access-Control-Allow-Origin", "*");
    res.header("Access-Control-Allow-Credentials", "true");
    res.setHeader("Access-Control-Allow-Headers", "Authorization, Access-Control-Allow-Headers, Origin,X-Requested-With,Content-Type,Accept,content-type,application/json");
    res.setHeader("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS ")
    if ('OPTIONS' == req.method) {
      res.send(200);
    } else {
      next();
    }
  })

With no results, I tried this setting on my Cors

app.use(cors( {
    credentials: 'true',  
    origin: '*', 
    methods: 'GET, POST, PUT, DELETE, OPTIONS', 
    allowedHeaders: 'Authorization, Access-Control-Allow-Headers, Origin,X- Requested-With,Content-Type,Accept,content-type,application/json'
  }));

As I was still getting the error, I found in the forum Netlify (https://community.netlify.com/t/how-handle-cors-error-in-netlify/2721/8) the following suggestion: Create a file netlify.toml and try the following configuration:

[[redirects]]
  from = "/api/*"
  to = "https://backend-app.herokuapp.com/clientes/api/:splat"
  status = 200
# SPA router support.
[[redirects]]
  from = "/*"
  to = "/index.html"
  status = 200

But I still get the error of CORS. I believe this mistake is responsible for me not being able to be redirected to my path with this.$router.push("/tabela") when I submit my form to create a customer. That is, I type the customer data in the form and click register, with the console open I see that you get status pending and after a few seconds I get the error from CORS but the client was saved in my database. It is worth saying that all operations CRUD are working normally. Just at the time of creating a new client I have problems, the client is created but I am not redirected to my path because I get the error No 'Access-Control-Allow-Origin' header is present on the requested Resource

Does anyone know how to fix this problem?