Invalid options object. Dev Server has been initialized using an options object that does not match the API schema

Here is a workaround. Delete "proxy": "http://localhost:6000". Install package http-proxy-middleware with command npm install http-proxy-middleware --save. Create a file setupProxy.js inside your src folder or the root of your folder. Add these lines inside:

const { createProxyMiddleware } = require('http-proxy-middleware');

module.exports = function(app) {
  app.use(
    '/api',
    createProxyMiddleware({
      target: 'http://localhost:6000',
      changeOrigin: true,
    })
  );
};

Now, run your app. It should work.