Why INSERT INTO query not working with mysqli? [duplicate]

I'm making a small website, that works like diary, just for practice. I work with PHP and MYSQL. I've just started, I don't have any design or so, just the PHP-code and the form, thats the Code:

<?php

 session_start();

  if(array_key_exists("submit",$_POST))
  {
      
      $link = mysqli_connect("abcdefg", "abcdefg", "abcdefg", "abcdefg");
      
      if(mysqli_connect_error()){
          die("Connection to the database could not be established.");
          
    }
      
      $error = "";
      
      if(!$_POST['email'])
      {
            
            $error .= "The email address is missing, please enter an email address.<br>";
            
      }
        
        
      if(!$_POST['password'])
      {
            
            $error .= "The password is missing, you need a password.<br>";
            
        } 
        
        if($error != "")
        {
            
            $error = "<p>There was an error in your form. </p>".$error;
           
        }
      else 
      {
            
                    
                   $query = "SELECT id FROM sd_users WHERE email = '".mysqli_real_escape_string($link, $_POST['email'])."' LIMIT 1";

                    $result = mysqli_query($link, $query);

                    if(mysqli_num_rows($result) > 0)
                    {

                        $error = "This email is already registered.";
                        

                    }
                    else
                    {

                        $query = "INSERT INTO sd_users (email,  password) VALUES ('".mysqli_real_escape_string($link, $_POST['email'])."',
                        '".mysqli_real_escape_string($link, $_POST['password'])."')";
                        
                        mysqli_query($link, $query);

                        if(!mysqli_query($link, $query))
                        {

                            $error = "<p>You could not register, please try again.</p>";   

                        }
                        else
                        {
                            
                            $query = "UPDATE `sd_users` SET password = '".md5(md5(mysqli_insert_id($link)).$_POST['email'])."'WHERE id = ".mysqli_insert_id($link)." LIMIT 1";
                            
                            mysqli_query($link, $query);
                            
                            $_SESSION['id'] = mysqli_insert_id($link);
                            
                            if($_POST['stayLoggedIn'] == '1')
                            {
                             
                             setcookie("id", mysqli_insert_id($link), time() + 60*60*24*365*100);
                                
                            }
                            
                  echo "You have successfully registered!";
                  
                       }
              
                  }
    }
      
  }

?>

<form method = "post">
    
    <div id ="error"><?php echo $error; ?></div>

   <input type = "email" name = "email" placeholder = "Your emailadress">
   
   <input type = "password" name = "password" placeholder = "Password">

   <input type = "checkbox" name = "stayLoggedIn" value = 1>

  <input type = "submit" name = "submit" placeholder = "Sign Up!">



</form>

If you have a look at this part:

else
{

    $query = "INSERT INTO sd_users (email,  password) VALUES ('".mysqli_real_escape_string($link, $_POST['email'])."',
    '".mysqli_real_escape_string($link, $_POST['password'])."')";
    
    mysqli_query($link, $query);

    if(!mysqli_query($link, $query))
    {

        $error = "<p>You could not register, please try again.</p>";   
 
    }

you can find my problem: The INSERT INTO... line doesn't work, and I don't know why. Every time when I try to "sign up", it says "You could not register, please try again."

(I changed the informations at $link, so you can't use it.

source https://stackoverflow.com/questions/71285977/why-insert-into-query-not-working-with-mysqli