Подскажите, почему может не работать валидация (jQuery Validation plugin)?

$(document).ready(() => {
    tableRenderer();
});
​
function tableRenderer() {
    $.ajax({
        method: "GET",
        url: "/users",
        dataType: "json",
        success: (json) => {
            console.log(json);
​
            for (let i = 0; i < json.length; i++) {
​
                // object's fields
                let id = json[i].id;
                let username = json[i].username;
                let password = json[i].password;
​
                // fields parsing
                let tr = $('<tr/>').addClass(`table-row-${id}`);
                tr.append(`
                        <td> ${id} </td>
                        <td> ${username} </td>
                        <td> ${password} </td>
                        <td>
                            <button type="submit" class="btn btn-info btn-sm" id="buttonEditUser" 
                                    onclick="editButton(${id}, '${username}', '${password}')"> edit </button>
                            <button type="submit" class="btn btn-danger btn-sm" id="buttonEditUser"
                                    onclick="deleteButton(${id}, '${username}')"> delete </button>
                        </td>`);
                $('table').append(tr);
            }
​
        }
    });
}
​
function addButton() {
    $('#addForm').validate({
        rules: {
            username: {
                required: true,
                remote: {
                    url: "/users/is_free",
                    type: "GET",
                    cache: false,
                    dataType: "json",
                    parameterData: {
                        username: function () {
                            return $("#username").val();
                        },
                    }
                }
            },
            password: {
                required: true,
                minlength: 3
            }
        },
        messages: {
            username: {
                required: "Nickname required! ",
                remote: "User with this nickname already exists!"
            },
            password: {
                required: "Password required",
                minlength: "Password must contain >= 3 symbols"
            }
        },
        submitHandler: function () {
            $.ajax({
                method: "POST",
                url: `/users`,
                dataType: 'json',
                contentType: "application/json; charset=utf-8",
                data: JSON.stringify({
                    username: $('#addForm #username').val(),
                    password: $('#addForm #password').val(),
                })
            });
​
            window.location = "http://localhost:8080/";
            location.reload();
        }
    });
}



<div class="container" th:fragment="addUserForm">

    <div class="card rounded-0 border-top-0">
        <h5 class="card-header"> Add new user </h5>
        <div class="card-body">
            <!-- ADD USER FORM -->
            <div class="container d-flex justify-content-center">
                <form id="addForm">
                    <!-- TEXT FIELD GROUP -->
                    <div class="form-group">
                        <label class="d-flex justify-content-center font-weight-bold" for="username"> Username </label>
                        <input class="form-control form-control-sm"
                               type="text" id="username" placeholder="nickname">
                    </div>
                    <div class="form-group">
                        <label class="d-flex justify-content-center font-weight-bold" for="password"> Password </label>
                        <input class="form-control form-control-sm"
                               type="password" id="password" placeholder="password">
                    </div>
                    <br>
                    <div class="container d-flex justify-content-center">
                        <button type="button" class="btn btn-success" id="buttonCreateUser" onclick="addButton()">
                            Create user
                        </button>
                    </div>
                </form>
            </div>
        </div>
    </div>
</div>

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