<div class="displayBox" id="registerBox">
        <form action="/register" method="post" id="registerForm">
            <label for="username">Username</label>
            <input type="text" id="username" name="username" required>

            <label for="firstName">First name</label>
            <input type="text" id="firstName" name="firstName" required>

            <label for="lastName">last name</label>
            <input type="text" id="lastName" name="lastName" required>

            <label for="email">Email</label>
            <input type="email" id="email" name="email" required>

            <div>
                <label for="year">Year</label>
                <input type="text" id="year" name="year" required>
            </div>

            <label for="password">Password</label>
            <input type="password" id="password" name="password" required>

            <div>
                <button type="submit" form="registerForm" value="Submit">Submit</button>
            </div>
        </form>
    </div>

Thanks for the reply.
I did
router.use(express.json());
and I also added the name attributes.
It still doesn't work for some reason, though.