body{
    background-color: #354457;
    color: #fff;
}
.title{
    margin-top: 30px;
}
#container{
    max-width: 150px;
    margin: 0 auto;
}
.login_container{
    display: flex;
    flex-wrap: wrap;
    justify-content: center;
    align-items: center;
}<div id="container">
    <div class="login_container">
        <p class="title">WELCOME</p>
        <p class="subtitle">LET'S GET STARTED</p>
        <input type="text" value="" placeholder="Username" required>
        <br>
        <input type="password" placeholder="Password">
        <br>
        <button>Login</botton>
    <div>
<div>When I use the flexbox, I have no idea why its properties cannot work. It seems "justify-content" is working but not the "align-items". Can anyone help me out?
 
     
    