req.body not functioning in Mongo database

I am new to both Mongo and Stackoverflow, and I have a question so basic that I can’t find it anywhere online.
I am trying to query a Mongo database with the following code in an index.js file:

app.post('/users', async (req, res) => {
    await Users.findOne({ Username: req.body.Username })
        .then((user) => {
            if (user) {
                return res.status(400).send(req.body.Username + 'already exists');
            } else {
                Users
                    .create({
                        Username: req.body.Username,
                        Password: req.body.Password,
                        Email: req.body.Email,
                        Birthday: req.body.Birthday
                    })
                    .then((user) => {res.status(201).json(user) })
                .catch((error) => {
                    console.error(error);
                    res.status(500).send('Error' + error);
                })
            }
        })
        .catch((error) => {
            console.error(error);
            res.status(500).send('Error' + error);
        });
});

This should allow me to create a new “user” based on the following schema in a models.js file:

let userSchema = mongoose.Schema({
    Username: {type: String, required: true},
    Password: {type: String, required: true},
    Email: {type: String, required: true},
    Birthday: Date,
    FavoriteMovies: [{type: mongoose.Schema.Types.ObjectId, ref: 'Movies'}]
});

However, attempting to query this in Postman crashes my server and displays the following error (that the Username part of req.body.Username is undefined):
Terminal Error

Any time I try to use req.body.Username, it crashes and says that Username is undefined. However, all users in the Users model have Usernames; also, using req.params.Username functions as normal. I have been running into this problem for days now and can’t seem to solve it.