req.user.save is not a function

[Uhilasb]

Ive took every step that you did but yet im getting this error and i just cant find where is the problem?

const mongoose = require('mongoose');
const { Schema } = mongoose;

const UserSchema = new Schema({
googleId: String,
credits: { type: Number, default: 0}
});

mongoose.model('users', UserSchema);

//Route code
const keys = require('../config/keys');
const stripe = require('stripe')(keys.stripeSecretKey);
const User = require('../models/User');

module.exports = app => {
app.post('/api/stripe', async (req, res) => {
const charge = await stripe.charges.create({
amount: 500,
currency: 'usd',
description: '$5 for 5 credits',
source: req.body.id
});
req.user.credits += 5;
const user = await req.user.save();

res.send(user);

});
};

//Index file
const express = require('express');
const mongoose = require('mongoose');
const keys = require('./config/keys');
const bodyParser = require('body-parser');
const passport = require('passport');
require('./models/User');
require('./services/passport');

mongoose.connect(keys.mongoURI);
const app = express();

app.use(bodyParser.json());
app.use(cookieSession({
maxAge: 30 * 24 * 8 * 8 * 1000,
keys: [keys.cookieKey]
}));
app.use(passport.initialize());
app.use(passport.session());

require('./routes/authRoutes')(app);
require('./routes/billingRoutes')(app);

const PORT = process.env.PORT || 5000;
app.listen(PORT);
console.log('The server is running!');

[SudhirChavda19]

you solved this error ??
if solved then please tell me
i also face same error