Form etiketini kapatmamışsınız bundan kaynaklı olabilir (</form>).
    <body id="register_bg">
    <div id="register">
            <div class="sign-in-wrapper">
        <aside>
            <figure>
                <a href="index.html"><img src="img/logo_sticky.svg" width="140" height="35" alt=""></a>
            </figure>
            <div class="access_social">
                    <a href="#0" class="social_bt facebook">Login with Facebook</a>
                    <a href="#0" class="social_bt google">Login with Google</a>
                </div>
            <div class="divider"><span>Or</span></div>
 
     
 
             
                 
             
 
 
     
            <?=@$cevap?>
            <form method="post">
                 
                <div class="form-group">
                    <input class="form-control" type="text" name="kadi"  placeholder="Kullanıcı Adınız">
                    <i class="icon_mail_alt"></i>
                </div>
                <div class="form-group">
                    <input  class="form-control" type="password" name="pass" id="password" placeholder="Şifreniz">
                    <i class="icon_lock_alt"></i>
                </div>
                <div class="clearfix add_bottom_15">
                    <div class="checkboxes float-left">
                        <label class="container_check">Beni hatırla
                          <input type="checkbox">
                          <span class="checkmark"></span>
                        </label>
                    </div>
                    <div class="float-right"><a id="forgot" href="#0">Forgot Password?</a></div>
                </div>
                 <input type="submit"  value="Giriş Yap" class="btn_1 full-width mb_5">
                  
                <div class="text-center mt-2"><small>Don't have an acccount? <strong><a href="register.html">Sign Up</a></strong></small></div>
                 </form>
                </div>
         
            <div class="copy">© 2020 FooYes</div>
        </aside>
    </div>
         
    <!-- /login -->
     
    <!-- COMMON SCRIPTS -->
    <script src="js/common_scripts.min.js"></script>
    <script src="js/common_func.js"></script>
    <script src="assets/validate.js"></script>
   
</body>
</html>