SyntaxError: Unexpected token 'n', "#" is not valid JSON (React Native app)

Question

I am trying to create an app using react native as the front end and node.js + express + mongoDB as the backend. I have no issues viewing the data from mongoDB in react native, my issue is when I try posting data to the database, every time I get the following error in both the backend and frontend.

Frontend

Here is CreatePostScreen.js in react native:

import { create } from "apisauce";
const apiClient = create({
  baseURL: "My IP Address : port number",
});

const addPost= (post) => {
  const data = new FormData();
  data.append("name", post.name);
  data.append("title", post.title);

  return apiClient.post('/post/create', data);
};


const handleSubmit = async (data) => {
    const result = await addPost(data);

    if (!result.ok) return alert(result.originalError);
    alert("done");
};

<AppForm
  initialValues={{
    name: "",
    title: "",
  }}
  onSubmit={handleSubmit}
  validationSchema={validationSchema}>
    <AppFormField name={"name"} placeholder="Name" />
    <AppFormField name={"title"} placeholder="Title" />
    
    <SubmitButton title={"Create Post"} />
</AppForm>

Here is the AppForm.js component

function AppForm({ initialValues, onSubmit, validationSchema, children }) {
  return (
    <Formik
      initialValues={initialValues}
      onSubmit={onSubmit}
      validationSchema={validationSchema}>
      {() => <>{children}</>}
    </Formik>
  );
}

Here is the AppFormField.js component

function AppFormField({ name, ...otherProps }) {
  const { setFieldTouched, setFieldValue, errors, touched, values } =
    useFormikContext();

  return (
    <>
      <TextInput
        onChangeText={(text) => setFieldValue(name, text)}
        value={values[name]}
      />
      <ErrorMessage error={errors[name]} visible={touched[name]} />
    </>
  );
}

Backend

Here is the postModel.js code

const mongoose = require("mongoose");

const PostSchema = new mongoose.Schema({
  name: { type: String, required: true },
  title: { type: String, required: true },
});

module.exports = mongoose.model("Post", PostSchema );

Here is the postController.js code

const Post = require("../models/postModel");
const mongoose = require("mongoose");
const dotenv = require("dotenv");
dotenv.config();

mongoose
  .connect(process.env.MONGO_URL)
  .then(() => console.log("db connected"))
  .catch((err) => console.log(err));

module.exports = {
  createPost: async (req, res) => {
    try {
      const p = new Post(req.body);
      const val = await p.save();
      res.status(200).json(val);
    } catch (error) {
      res.status(500).json("Failed to create a new post");
    }
  },

Here is the postRouter.js code

const postController = require("../controllers/postController ");
const express = require("express");
const router = express.Router();

router.use(express.urlencoded({ extended: true }));
router.use(express.json());

router.get("/", structureController.getAllStructures);
router.get("/:id", structureController.getStructure);
router.get("/search/:key", structureController.searchStructure);

router.post("/create",postController.createPost);

Here is the index.js code

const express = require("express");
const app = express();
const postRouter = require("./routers/post");

app.use("/structures", structureRouter);

app.listen(process.env.PORT, "My IP Address", () =>
  console.log(`App listening on port ${process.env.PORT}!`)
);

Errors

Here is the error that shows up in react native:

Here is the error that shows up in the backend

SyntaxError: Unexpected token 'n', "#" is not valid JSON
    at JSON.parse (<anonymous>)
    at createStrictSyntaxError (node_modules\express\node_modules\body-parser\lib\types\json.js:160:10)
    at parse (node_modules\body-parser\lib\types\json.js:83:15)
    at node_modules\body-parser\lib\read.js:128:18
    at AsyncResource.runInAsyncScope (node:async_hooks:206:9)
    at invokeCallback (node_modules\raw-body\index.js:231:16)
    at done (node_modules\raw-body\index.js:220:7)
    at IncomingMessage.onEnd (node_modules\raw-body\index.js:280:7)
    at IncomingMessage.emit (node:events:514:28)
    at endReadableNT (node:internal/streams/readable:1376:12)

Can anyone please assist me on how to overcome this error?