Access-Control-Allow-Origin的CORS错误*-NodeJS和Angular

I'm doing a web app with Angular and when I connect the backend with the front it gives me a CORS error: Access to XMLHttpRequest at 'http://localhost:3700/api/save-project' from origin 'http://localhost:4200' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: It does not have HTTP ok status.

我知道CORS是什么，但我不知道为什么会出现此错误，因为在后端的app.js中，我有阻止CORS的代码

如何解决此问题？

码：

app.js     “使用严格”

var express = require('express');
var bodyParser = require('body-parser');

var app = express();

// cargar archivo Rutas
var routes = require('./routes/product');

// Middlewares
app.use(bodyParser.urlencoded({extended:false}));
app.use(bodyParser.json()); // Content-Type: image/jpg

// Configurar cabeceras y cors
app.use((req, res, next) => {
    res.header('Access-Control-Allow-Origin', '*');
    res.header('Access-Control-Allow-Headers', 'Authorization, X-API-KEY, Origin, X-Requested-With, Content-Type, Accept, Access-Control-Allow-Request-Method');
    res.header('Access-Control-Allow-Methods', 'GET, POST, OPTIONS, PUT, DELETE');
    res.header('Allow', 'GET, POST, OPTIONS, PUT, DELETE');
    next();
});

// Rutas
app.use('/api', routes);

// exportar
module.exports = app;

Angular中的组件：

import { Component, OnInit } from '@angular/core';
import { ProductService } from 'src/app/services/products.service';
import { Product } from 'src/app/models/product';
import { HttpClient } from '@angular/common/http';

@Component({
  selector: 'crear',
  templateUrl: './create.component.html',
  styleUrls: ['./create.component.css'],
  providers: [HttpClient, ProductService]
})
export class CreateComponent implements OnInit {
  public producto: Product;
  public product: any;

   constructor(
    //,
    //private http: HttpClient
    private _productService: ProductService
  ) { 
    this.producto = new Product('','','','',2,2);
  }

  ngOnInit(): void {
  }

  onSubmit(form){
    this._productService.saveProduct(this.producto).subscribe(
      response => {
        if(response.producto){

          form.reset();

          this.product = this.producto._id;
        }else{
        alert("No se ha podido");
      };
      } 
    )
  }

}

服务：

import { Injectable } from "@angular/core";
import { Observable } from 'rxjs';
import { HttpClient, HttpHeaders } from '@angular/common/http';
import { Product } from '../models/product';
import { Global } from './global.service';

@Injectable()
export class ProductService{
    public url: String;

    constructor(
        private _http: HttpClient
    ){
        this.url = Global.url;
    }

    saveProduct(product: Product): Observable<any>{
        var params = JSON.stringify(product);
        let headers =  new HttpHeaders().set('Content-Type', 'application/json');

        return this._http.post(this.url+'save-project', params, {headers: headers});
    }
}

谢谢大家