Send data via POST - MEAN STACK

When I pass a null value I get the backend function return, but when the value is not null, I get a bad request (400) error. See what I'm doing, what could be wrong?

Error:

  

POST link 400 (Bad Request)

     

Failed to load link : In 'Access-Control-Allow-Origin' header is present on the requested resource. Origin ' link ' is therefore not allowed access. The response had HTTP status code 400.

My code:

Factory angularjs:

function ApiExterna($http, consts) {

    function buscarCep(cep) { //cep é uma string
        $http.post('http://localhost:3003/api/buscarCep', cep)
            .then(resp => {
                console.log(resp)
                return null     
            }).catch(function (resp) {
        )
    }

    return { buscarCep }
}

Backend route:

const express = require('express')
const auth = require('./auth')   

module.exports = function(server) {

  const protectedApi = express.Router()
  server.use('/api', protectedApi)

  const ApiService = require('../api/apiExterna/apiService')
  protectedApi.post('/buscarCep', ApiService.buscarCep)    
}

Backend service:

const bodyParser = require('body-parser')

const buscarCep = (req, res, next) => {
    if (!req.body.cep) return res.send("hahahahah" + req.body)
    res.send('welcome' + req.body)  
}

module.exports = { buscarCep }

Any help? Thank you!