Javascript – How to send data to mongodb with fetch API and FormData

I want to send data to mongoDB with the fetch API and FormData, but i get an error:

POST https://xxxxxxxxxxxxxxxxxxxxxxxx/upload 500 (Internal Server Error)

My form data in EJS file is this:

    <div id="image_data">        
        <form action="/upload" method="post">
            <p class="title_content">Gebe hier Daten über dein Foto an:</p>
            <br>
            <input type="hidden" value=<%= username %> name="username" />
            <br>
            <input id="Base64IMG" type="hidden" value="" name="imgBase64" />
            <br>
            <label for="city">Ort: </label>
            <input id="city" placeholder="Ort" name="city" type="text" />
            <br />
            <label for="date">Aufgenommen am: </label>
            <input id="date" placeholder="Datum" name="date" type="date" />
            <br />
            <textarea id="textarea" rows ="5" cols="40" placeholder="Beschreibe kurz dein Bild" name="description" ></textarea>
            <br>
            <input class="sub" type="submit" value="upload"/>
        </form>
</div>

my Javascript, where the fetch API is:

function toServer(formData){
   fetch('/upload', {
      method: 'POST',
      body: formData
    })
      .then(function(response) {
        if (response.ok) {
          console.log('POST-Request erfolgreich abgesendet');
          console.log(response)
          alert("hat geklappt")
          // Weitere Verarbeitungsschritte nach erfolgreichem POST-Request
        } else {
          console.log('Fehler beim Absenden des POST-Requests');
        }
      })
      .catch(function(error) {
        console.log('Fehler beim Absenden des POST-Requests:', error.message);
      });
}

const form = document.querySelector('#image_data > form')

form.addEventListener('submit', function(event) {
   event.preventDefault()
   const formData = new FormData(form)
   
   if (navigator.onLine) {
      toServer(formData)
    } else {
.
.
.
}}

and my /upload route:

router.post("/upload", async (req,res) => {
  try {
    await img.create(req.body)
   if(true){
    res.status(200).json({message:true})
   }else{
    res.status(200).json({message:false})
   }
  } catch (error) {
      console.log(error.message);
      res.status(500).json({message: error.message})
  }
  console.log("upload wurde durchgeführt")
})

img is importet from a mongoose schema from const img = require("../database/picture");

Can somebody help me, what im doing wrong. 🙂

EDIT:
in the console of my server i get this output:

pictureData validation failed: description: Path `description` is required., date: Path `date` is required., city: Path `city` is required., imgBase64: Path `imgBase64` is required., username: Path `username` is required.

My mongoose Scheema is this:

const mongoose=require("mongoose")

const IMGSchema=new mongoose.Schema({
    username:{
        type:String,
        required:true
    },
    imgBase64:{
        type:String,
        required:true
    },
    city:{
        type:String,
        required:true
    },
    date:{
        type:String,
        type:Date,
        required:true
    },
    description:{
        type:String,
        required:true
    },
})

const img=new mongoose.model("pictureData", IMGSchema)

module.exports=img
```´