Warning: mysqli_fetch_assoc() expects parameter 1 to be mysqli_result, boolean given in C:\wamp64\www\test\personal details functions.php on line 71 hi i keep getting this error and i cant solve it

<?php
    session_start();
// connect to database
$db = mysqli_connect('localhost', 'root', '12345', 'personal details');

// variable declaration
$username = "";
$Email    = "";
$Phonenumber = "";
$country = "";
$errors = array();

// call the register() function if register_btn is clicked
if (isset($_POST['register_btn'])) {
      register();
}

// REGISTER USER
function register(){
      // call these variables with the global keyword to make them available in function
      global $db, $errors, $username, $Email , $Phonenumber , $country;

      // receive all input values from the form. Call the e() function
    // defined below to escape form values
      $username    = e($_POST['username']);
      $Email       = e($_POST['Email']);
      $Phonenumber = e($_POST['Phonenumber']);
    $country     = e($_POST['country']);
      // form validation: ensure that the form is correctly filled
      if (empty($username)) {
            array_push($errors, "username is required");
      }
      if (empty($Email)) {
            array_push($errors, "Email is required");
      }
      if (empty($Phonenumber)) {
            array_push($errors, "Phonenumber is required");
    }
      if (empty($country)) {
            array_push($errors, "country is required");
    }
      // register user if there are no errors in the form
      

            if (isset($_POST['personal details'])) {
                  $user_type = e($_POST['personal details']);
                  $query = "INSERT INTO users (username, Email, Phonenumber, country )
                                VALUES('$username', '$Email', '$Phonenumber', '$country' )";
                  mysqli_query($db, $query);
                  $_SESSION['success']  = "New user successfully created!!";
                  header('C:\wamp64\www\test\HOME PAGE 1.HTML');
            }else{
                  $query = "INSERT INTO personal details (username, Email, Phonenumber, country )
                                VALUES('$username', '$Email', '$Phonenumber', '$country' )";
                  mysqli_query($db, $query);

                  // get id of the created user
                  $logged_in_personaldetails_id= mysqli_insert_id($db);

                  $_SESSION['personaldetails'] = getUserById($username); // put logged in user in session
                  $_SESSION['success']  = "New user successfully created!!";
                  header('HOME PAGE 1.HTML');
            }
      }
// return user array from their id
function getUserById($username){
      global $db;
      $query = "SELECT * FROM `personal details` WHERE username=" . $username;
      $result = mysqli_query($db, $query);

      $personaldetails = mysqli_fetch_assoc($result);
      return $username;
}

// escape string
function e($val){
      global $db;
      return mysqli_real_escape_string($db, trim($val));
}

function display_error() {
      global $errors;

      if (count($errors) > 0){
            echo '<div class="error">';
                  foreach ($errors as $error){
                        echo $error .'<br>';
                  }
            echo '</div>';
      }
}

function isLoggedIn()
{
      if (isset($_SESSION['user'])) {
            return true;
      }else{
            return false;
      }
}
?>