JSFiddle - React, Tailwind, and code Playground

HTML

Country:<br />
<select id="drop1">
    <option value="">Select Country</option>
    <option value="c1">Country 1</option>
    <option value="c2">Country 2</option>
</select>
<br />
City:<br />
<select id="drop2" disabled >
    <option value="">Select Country</option>
    <option value="c1">Country 1</option>
    <option value="c2">Country 2</option>
</select>
<div class="mydiv">
<form class="form-signin ajax" method="post" action="ajax.php" novalidate="novalidate">
<div class="main-error alert alert-error hide"></div>
<input name="username" class="field" placeholder="Ваш E-mail" autofocus="" type="text">
<input name="names" class="field" placeholder="Ваше имя" type="text">
<input name="tel" class="field" placeholder="Телефон 10 цифр" type="text">

<select name="status" type="text" class="field">
<option value="Собственник" id="sobstv" onclick="check1()" class="field">Собственник </option>
<option value="Житель" class="field">Житель</option>
<option value="Арендатор" class="field">Арендатор</option>
</select>

<input name="kv" class="field" placeholder="№ Квартиры" type="text">


<input name="area" class="field" placeholder="Кол-во метров" type="text">
<input name="share" class="field" disabled="" placeholder="Доля" type="text">

<input name="password1" class="field" placeholder="Пароль" type="password">
<input name="password2" class="field" placeholder="Повтор пароля" type="password">

<input id="politics" onclick="check();" value="" autocomplete="off" type="checkbox">
Нажимая на кнопку "Регистрация", я даю согласие на обработку персональных данных.

<input name="act" value="register" type="hidden">
<input class="button" id="regsubmit" name="submit" value="Регистрация" disabled="" type="submit">
</form>

<script>
function check() {
var submit = document.getElementsByName('submit')[0];
if (document.getElementById('politics').checked)
submit.disabled = '';
else
submit.disabled = 'disabled';
}
</script>

<script>
function check1() {
var text =...

JavaScript

$("#drop1").change(function(){
    var country = $(this).val();

    if(!country){
        $("#drop2").attr("disabled", true);
        return false;
    }

    $("#drop2").attr("disabled", false);
});