Warning: Cannot modify header information - headers already sent by

Question

I want to add some "dependencies" (Bootstrap and CanvasJS) to my WordPress website. Therefore I wrote such code in functions.php file:

function basic_enqueue_style()
{
    /*Bootstrap*/
    wp_enqueue_script('popper', 'https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js', ['jquery']);
    wp_enqueue_script('bootstrap', 'https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js', ['jquery', 'popper']);
    wp_enqueue_style('bootstrap', 'https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css');

    /*Personal CSS*/
    wp_enqueue_style('main', get_template_directory_uri() . '/css/main.css');

    /*CanvasJS*/
    wp_enqueue_script('canvasjs', get_template_directory_uri() . '/js/canvasjs.min.js');
}

add_action('wp_enqueue_scripts', 'basic_enqueue_style');

As "dependencies" they should be loaded from header not footer thus I did not indicated $in footer as true (default false) parameter.

Documentation says:

wp_enqueue_script( $handle, $src, $deps, $ver, $in_footer);

The error information I get after adding "dependencies":

Shall I change something in index.php file? Which now looks like this:

<?php get_header(); ?>

<?php if (is_user_logged_in()): ?>
    <?php  $expensesFormPage = get_page_by_title('formularz wydatkow'); ?>
    <?php  wp_redirect(get_permalink($expensesFormPage->ID)); ?>
<?php else: ?>
    <a href="<?php echo wp_registration_url(); ?>">Zarejestruj sie!</a>
    <?php wp_login_form(); ?>
<?php endif; ?>

<?php get_footer(); ?>

Many thanks for any help!!

Answer 1

OK. Thanks to @Milo and @Krzysiek Dróżdż I changed the code in index.html file and it works now.

I put get_header() after redirection method.

<?php if (is_user_logged_in()): ?>
    <?php $expensesFormPage = get_page_by_title('formularz wydatkow'); ?>
    <?php wp_redirect(get_permalink($expensesFormPage->ID)); ?>
<?php else: ?>
    <?php get_header(); ?>
    <h1>Aby korzystać z aplikacji musisz się zarejestrować.</h1>
    <a href="<?php echo wp_registration_url(); ?>">Zarejestruj się!</a>
    <?php wp_login_form(); ?>
<?php endif; ?>

<?php get_footer(); ?>