Cannot read properties of undefined (reading 'name') when posting to mongoose

ERROR:

Cannot read properties of undefined (reading 'name')

router.post("/", async (req, res) => {
const newItem = new items({
    name: req.body.name
})

try {
    const submitted = await items.save();
    console.log(submitted);
}
catch (err) {
    console.log(err)
}
})

Schema:

const mongoose = require("mongoose");

 const pieces = new mongoose.Schema({
    name: String
 })

 module.exports = mongoose.model("items", pieces);




//index
const express = require("express");
const app = express();
const mongoose = require("mongoose");
require("dotenv/config");

const product = require("../backend/Router/productRoutes");

app.use("/add", product);
app.use(express.json());
app.use(express.urlencoded({ extended: true }));

mongoose.connect(process.env.MONGO_DB, () => {
    console.log("DB CONNECTED")
});

app.listen(3500, () => {
    console.log("server up");
});

this same code works in another project so I'm confused as to what's going on here. I tried almost every suggestion but nothing works.