我有一个Next.js应用程序。我有一个错误与Mongodb。我不能解决这个问题一段时间。这就是为什么我想问你们。我希望我们可以解决它。这对我来说非常重要。首先我会给予你我的server.js代码，之后我会给你的错误消息从控制台。
Server.js文件代码：

const express = require('express')
const next = require('next')
const mongoose = require('mongoose')
require('dotenv/config')
const dev = process.env.NODE_ENV !== 'production'

const app = next({ dev })
const handle = app.getRequestHandler()
const port = process.env.SERVER_PORT || 3000

mongoose.connect(process.env.DATABASE,{
  useNewUrlParser: true,
  useUnifiedTopology: true,
})
.then(() => {
  console.log('DB connected!')

  const port = process.env.PORT || 3000

  app.prepare().then(()=>{
    const server = express()
  
    //ROUTES
    server.use(express.json())
    server.get('*', (req, res) => handle(req, res))
  
    server.listen(port, (err) => {
      if(err) throw err
      console.log(`Server listen on http://127.0.0.1:${port}`)
    })
  
  })

})
.catch((err) => {
  console.log('DB ERROR:', err)
})

控制台上的错误消息：

DB ERROR: MongooseError: The `uri` parameter to `openUri()` must be a string, got "undefined". Make sure the first parameter to `mongoose.connect()` or `mongoose.createConnection()` is a string.
    at NativeConnection.Connection.openUri (/home/ubuntu/nodeApps/mywebsite-next/node_modules/mongoose/lib/connection.js:684:11)
    at /home/ubuntu/nodeApps/mywebsite-next/node_modules/mongoose/lib/index.js:332:10
    at /home/ubuntu/nodeApps/mywebsite-next/node_modules/mongoose/lib/helpers/promiseOrCallback.js:32:5
    at new Promise (<anonymous>)
    at promiseOrCallback (/home/ubuntu/nodeApps/mywebsite-next/node_modules/mongoose/lib/helpers/promiseOrCallback.js:31:10)
    at Mongoose._promiseOrCallback (/home/ubuntu/nodeApps/mywebsite-next/node_modules/mongoose/lib/index.js:1158:10)
    at Mongoose.connect (/home/ubuntu/nodeApps/mywebsite-next/node_modules/mongoose/lib/index.js:331:20)
    at Object.<anonymous> (/home/ubuntu/nodeApps/mywebsite-next/server.js:11:10)
    at Module._compile (node:internal/modules/cjs/loader:1101:14)
    at Object.Module._extensions..js (node:internal/modules/cjs/loader:1153:10)

我的env文件中有Mongodb的链接，可能是这种情况导致的错误？

DATABASE=mongodb+srv://myusername:mypassword@cluster0.u4slu.mongodb.net/mydbusername?retryWrites=true&w=majority