MongoDB: How to get and display items by userID

I’m trying to get and display all entries in an ‘item’ collection that contain and are listed by a user/userID.

item_controller.js:

const Item = require('../models/item_schema')

const getUserItems = (req, res) => {
    Item.findById(req.params.userID).populate('categoryID').populate('qualityID').populate('userID')
    .then((data) => {
        if (data) {
            res.status(200).json(data)
        } else {
            res.status(404).json(`Item from user id ${req.params.userID} not found.`)
        }
    })
}

module.exports = {
    getUserItems
}

item_schema.js:

const { Schema, model} = require('mongoose')

const itemSchema = new Schema({
    title: {
        type: String,
        required: [true, 'Title field is required.']
    },
    description: {
        type: String,
        required: [true, 'Description field is required.']
    },
    userID: {
        type: Schema.Types.ObjectId,
        ref: "User",
        required: [true, 'User field is required']
    },
    categoryID: {
        type: Schema.Types.ObjectId,
        ref: "Category",
        required: [true, 'Category field is required']
    },
    qualityID: {
        type: Schema.Types.ObjectId,
        ref: "Quality",
        required: [true, 'Quality field is required']
    },
    price: {
        type: Number
    },
    claimed:{
        type: Boolean
    }
}, {
    timestamps: true
})

module.exports = model('Item', itemSchema)

server.js:

const express = require('express')
const cors = require('cors')
const jwt = require('jsonwebtoken')

require('dotenv').config()
require('./db')()

const { getUserItems } = require('./controllers/item_controller')

const port = process.env.PORT || 3000

const app = express()
app.use(cors())
app.use(express.json())

app.use((req, res, next) => {
    if (req.headers && req.headers.authorization && req.headers.authorization.split(' ')[0] === 'Bearer') {
        jwt.verify(req.headers.authorization.split(' ')[1], 'zero_waste_app', (err, decode) => {
            if (err) req.user = undefined
            req.use r = decode
            next()
        })
    } else {
        req.user = undefined
        next()
    }
})

// Item Routes 
app.get('items/user/:userID', getUserItems)

app.listen(port, () => {
    console.log(`Example app listening on port ${port}`)
})

I think my problem is in the item_controller, I’ve tried a few different ways, such as Item.find({userID: userID}), but I can’t get it to work. Does anyone know what I’m doing wrong and how I can fix it?