我正在开发一个应用程序与我自己的服务器，我配置我的cors与客户端主机只。一切似乎都很好，我可以请求数据库中的数据使用GET，但每当我试图POST或创建，我总是有“响应预检请求不通过访问控制检查：“请我做错了什么？以下是我的代码
我的服务器端

const allowedOrigins = [
    'http://localhost:3000/'
]
const CorsOptions = {
    origin: allowedOrigins,
    Credentials: true,
    methods: ['GET', 'POST', 'PUT', 'PATCH', 'DELETE'],
    allowedHeaders: ['Content-Type'],
    optionsSuccessStatus: 204
}
const cors = require('cors')
const CorsOptions = require('./config/CorsOptions')
app.use(cors(CorsOptions))
app.use(express.urlencoded({ extended: false }))
app.use(express.json())

我的客户端

const response = await fetch('http://localhost:8000/workout', {
            method: 'POST',
            body: JSON.stringify(data),
            headers: {
                'Content-Type':'application/json'
            },
        })
        const json = await response.json()
        if(!response.ok){
            errors(json.message)
        }
        if(response.ok){
            reset(data)
            console.log('New Workout added successfully', json)
        }

错误

Access to fetch at 'http://localhost:8000/workout' from origin 'http://localhost:3000' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: No 'Access-Control-Allow-Origin' header is present on the requested resource. If an opaque response serves your needs, set the request's mode to 'no-cors' to fetch the resource with CORS disabled.

GET请求与上面的所有这些设置完美地工作，但是由于某种原因POST或任何其他请求总是给我这个错误
我还没有做任何事情，我将感谢任何帮助。