Unable to post data to a collection in mongodb database

Question

I am working with mongodb right now for creating an android app using react native.My problem is i am not able to post data to collection in mongodb.

My app.js code

const express = require("express");
const app = express();
const morgan = require("morgan");
const mongoose = require("mongoose");
const cors = require("cors");
require("dotenv/config");
app.use(cors());
app.options("*", cors());
//middleware
app.use(express.json());
app.use(morgan("tiny"));
//Routes
const categoriesRoutes = require("./routes/categories");
const productsRoutes = require("./routes/products");
const usersRoutes = require("./routes/users");
const ordersRoutes = require("./routes/orders");
const api = process.env.API_URL;
app.use(`${api}/categories`, categoriesRoutes);
app.use(`${api}/products`, productsRoutes);
app.use(`${api}/users`, usersRoutes);
app.use(`${api}/orders`, ordersRoutes);
//Database
mongoose
  .connect(process.env.CONNECTION_STRING, {
    useNewUrlParser: true,
    useUnifiedTopology: true,
    dbName: "eshop-database",
  })
  .then(() => {
    console.log("Database Connection is ready...");
  })
  .catch((err) => {
    console.log(err);
  });

//Server
app.listen(3000, () => {
  console.log("server is running http://localhost:3000");
});

My categories.js:

const {Category} = require('../models/category');
const express = require('express');
const router = express.Router();

router.get('/', async (req, res) =>{
    const categoryList = await Category.find();
    if(!categoryList) {
        res.status(500).json({success: false})
    }
    res.send(categoryList);
})

router.post('/', async (req,res)=>{
    let category = new Category({
        name: req.body.name,
        icon: req.body.icon,
        color: req.body.color
    })
    category = await category.save();

    if(!category)
    return res.status(404).send('The category cannot be created!')

    res.send(category);
})

module.exports = router;

My category.js:

const mongoose = require('mongoose');
const categorySchema = mongoose.Schema({
    name: {
        type: String,
        required: true,
    },
    icon: {
        type: String,
    },
    color: {
        type: String,
    }
})

exports.Category = mongoose.model('Category', categorySchema);

Through postman tool i checked it but there is no error even though i am unable to post the data and in mongodb database it is displaying as Query Results: 0