$_POST not working. “Notice: Undefined index: SU_UserName

Question

When i try to send data using get method, everything will be ok, but if i tried to send data using post method, Undefined index will appear.

Error message: Undefined index: SU_UserName

this is the HTML code

      <form method="post" action="Login.php">
        <div class="form-group">
          <input type="text" name="SU_UserName" required>
        </div>
        <div class="form-group">
          <input type="text" name="SU_NickName">
        </div>
        <div class="form-group">
          <input type="email" name="SU_Email" required>
        </div>
        <div class="form-group">
          <input type="password" name="SU_Password" required>
        </div>

        <div class="loginbox">

          <input type="submit">
        </div>
      </form>

and this is Login.php

echo $_POST['SU_UserName'];

i also used isset(), var_dump($_Post), and the post array size was 0.