Trouble making API POST request with Next.js

I am a noob when it comes to anything related to backend, also new in Next.js and TypeScript.
I am trying to make a POST request to an API, which should receive formData and create a new listing based on that formData. I am trying to do this with Next.js server actions and route handlers.

Here is my code:

This is from the form component:

    const handleSubmit = async (e: React.FormEvent<HTMLFormElement>) => {
        e.preventDefault();

        const form = e.currentTarget as HTMLFormElement;
        const formData = new FormData(form);
        
        const address = formData.get("address");
        const mail = formData.get("mail");
        const region_id = selectedRegion?.id; //these come from states
        const agent_id = selectedAgent?.id;  //these come from states
        const city_id = selectedCity?.id;   //these come from states
        const area = formData.get("area");
        const price = formData.get("price");
        const type = formData.get("type");
        const cover = formData.get("cover"); // This is the file input

        try {
            console.log(address, mail, region_id, agent_id, city_id, area, price, type, cover);

            const newFormData = new FormData();

            newFormData.append("address", address as string);
            newFormData.append("mail", mail as string);
            newFormData.append("region_id", region_id as any);
            newFormData.append("agent_id", agent_id as any);
            newFormData.append("city_id", city_id as any);
            newFormData.append("area", area as string);
            newFormData.append("price", price as string);
            newFormData.append("type", type as string);
            newFormData.append("cover", cover as File); 
            
            await addListingAction(newFormData)
        } catch (error) {
            alert(`Error: ${error}`);
        }
    };

This is my server action:

    export async function addListingAction(formData: FormData) {
  try {
    await addListing(formData);
  } catch (error) {
    console.log(error);
  } finally {
    revalidatePath("/")
  }
}

my api.ts file:

export async function addListing(formData: FormData) {

  return await fetch(`${process.env.NEXT_PUBLIC_API_URL}/api/add-listing`, {
    method: 'POST',
    body: formData,
  })
    .then(response => response.json())
    .then(data => console.log(data))
    .catch(error => console.error('Error:', error));
}

and my route.ts file:

import { NextResponse } from 'next/server';

export async function POST(req: Request) {
  try {
    const formData = await req.formData();

    const address = formData.get("address");
    const mail = formData.get("mail");
    const region_id = formData.get("region_id");
    const agent_id = formData.get("agent_id");
    const city_id = formData.get("city_id");
    const area = formData.get("area");
    const price = formData.get("price");
    const type = formData.get("type");
    const cover = formData.get("cover") as File | null;

    const form = new FormData();
    if (address) form.append("address", address);
    if (mail) form.append("mail", mail);
    if (region_id) form.append("region_id", region_id);
    if (agent_id) form.append("agent_id", agent_id);
    if (city_id) form.append("city_id", city_id);
    if (area) form.append("area", area);
    if (price) form.append("price", price);
    if (type) form.append("type", type);
    if (cover instanceof File) form.append("cover", cover);

    const REDBERRY_API_TOKEN = process.env.REDBERRY_API_TOKEN;

    if (!REDBERRY_API_TOKEN) {
      throw new Error("Missing API token");
    }


    const response = await fetch('https://api.real-estate-manager.redberryinternship.ge/api/real-estates', {
      method: 'POST',
      headers: {
        'Authorization': `Bearer ${REDBERRY_API_TOKEN}`,
      },
      body: form,
    });


if (!response.ok) {
  let errorResponse;
  try {
    errorResponse = await response.json();
  } catch {
    errorResponse = await response.text();
  }
  console.error('HTTP error! Status:', response.status, 'Response:', errorResponse);
  return NextResponse.json(
    { message: 'API request failed', error: errorResponse },
    { status: response.status }
  );
}


  const listing = await response.json();

  return NextResponse.json({ listing }, { status: 201 });

  } catch (error) {
    const message = error instanceof Error ? error.message : 'An unexpected error occurred';
    console.error('Error creating listing:', message);
  return NextResponse.json(
    { message: 'Failed to create listing', error: message },
    { status: 500 }
  );
  }
}

The API documentation says that the curl request looks like this:

    curl -X 'POST' 
  'https://api.real-estate-manager.redberryinternship.ge/api/real-estates' 
  -H 'accept: application/json' 
  -H 'Authorization: Bearer API_TOKEN' 
  -H 'Content-Type: multipart/form-data' 
  -F 'region_id=1' 
  -F 'price=100000' 
  -F 'zip_code=0101' 
  -F 'area=100.5' 
  -F 'city_id=1' 
  -F 'address=example address' 
  -F 'agent_id=371' 
  -F 'bedrooms=3' 
  -F 'is_rental=0' 
  -F '[email protected];type=image/png' 
  -F 'description=house near metro station'

and the response looks like this:

"price": "100000",
  "zip_code": "0101",
  "area": "100.5",
  "city_id": "1",
  "address": "example description",
  "agent_id": "371",
  "bedrooms": "3",
  "is_rental": "0",
  "description": "house near metro station",
  "image": "https://api.real-estate-manager.redberryinternship.ge/storage/images/vZ6KRjLiBIoVqwl4GMniE598mAzno8wqzCkZyg3f.png",
  "created_at": "2024-09-15T16:43:21.000000Z",
  "id": 450

The error I am getting is:

Error creating listing: Unexpected token ‘<‘, “<!DOCTYPE “… is not valid JSON
POST /api/add-listing 500 in 931ms
{
message: ‘Failed to create listing’,
error: ‘Unexpected token ‘<‘, “<!DOCTYPE “… is not valid JSON’
}

Any help would be greatly appreciated.