req.body return ReadableStream in my route NextJS

I dont get it what is wrong of my code. I read and try other question like that but they didnt helped my

Is there any one who can help me ?

This is my route.js

import dbConnect from "@/lib/dbConnect";
import User from "@/models/User";
import { NextResponse, NextRequest } from "next/server";
import type { NextApiResponse, NextApiRequest } from "next";

export async function POST(req: NextApiRequest) {
  await dbConnect();
  let data = await req.body;
  console.log(data); //HERE ReadableStream { locked: false, state: 'readable', supportsBYOB: false } HERE

  try {
    const user = await new User(data);
    await user.save();

    return NextResponse.json({ user }, { status: 201 });
  } catch (error) {
    console.log("here 3");
    if (error instanceof Error) {
      return NextResponse.json(
        { error: error.message || "bilinmeyen bir hata" },
        { status: 500 }
      );
    } else {
      return NextResponse.json(
        { error: "bilinmeyen bir hata" },
        { status: 500 }
      );
    }
  }
}

This is my request;

try {
      const res = await fetch("http://localhost:3000/api/register", {
        method: "POST",
        headers: { "Content-Type": "application/json" },
        body: JSON.stringify(data),
      });

      const dataRes = await res.json();
      console.log("dataRes" + dataRes);
    } catch (error) {
      console.log(error);
    }