How to add structured data schema in array

I am not a professional in WordPress, but if you avoid global variables as much as you can, this will be good for your application. Just try the below code.

<?php
function generate_faq_schema () {
    if ( have_rows('faq') ) {
        $schema = [
            '@context'   => "https://schema.org",
            '@type'      => "FAQPage",
            'mainEntity' => array()
        ];
        while ( have_rows('faq') ) : the_row();
            $questions = [
                '@type'          => 'Question',
                'name'           => get_sub_field('question'),
                'acceptedAnswer' => [
                    '@type' => "Answer",
                    'text' => get_sub_field('answer')
                ]
            ];
            array_push($schema['mainEntity'], $questions);
        endwhile;
        echo '<script type="application/ld+json">'. json_encode($schema) .'</script>';
    }
}
add_action( 'wp_footer', 'generate_faq_schema', 100 );

?>

Also check for fourth argument of add_action here.

<section class="faq">
    <div class="container max-width px-5">
        <div class="row">
            <div class="col-lg-6 features-title-block">
                <h2 class="mpc-header"> <?php if ( get_field('faq_header') ) : ?>
                    <?php the_field('faq_header'); ?>
                    <?php endif; ?></h2>
                    <?php if ( get_field('faq_sub_text') ) : ?>
                    <div class="pb-5"><?php the_field('faq_sub_text'); ?></div>
                <?php endif; ?>
            </div>
            <div class="col-lg-6">
                <?php if ( have_rows('faq') ) : ?>
                <div class="accordion" id="accordionExample">
                    <?php while( have_rows('faq') ) : the_row(); ?>
                    <div class="card">
                        <div class="card-header" id="heading<?php echo get_row_index(); ?>">
                            <h2 class="mb-0">
                                <button class="btn btn-link d-inline-flex" type="button" data-toggle="collapse"
                                    data-target="#collapse<?php echo get_row_index(); ?>" aria-expanded="true"
                                    aria-controls="collapse<?php echo get_row_index(); ?>">

                                    <?php the_sub_field('question'); ?>

                                </button>
                            </h2>
                        </div>
                        <div id="collapse<?php echo get_row_index(); ?>" class="collapse"
                            aria-labelledby="heading<?php echo get_row_index(); ?>" data-parent="#accordionExample">
                            <div class="faq-content card-body">
                                <?php the_sub_field('answer'); ?>
                            </div>
                        </div>
                    </div>
                    <?php endwhile; ?>
                    <?php
                    $schema = array(
                    '@context'   => "https://schema.org",
                    '@type'      => "FAQPage",
                    'mainEntity' => array()
                    );
                    global $schema;
                    if ( have_rows('faq') ) {
                        while ( have_rows('faq') ) : the_row();
                            $questions = array(
                                '@type'          => 'Question',
                                'name'           => get_sub_field('question'),
                                'acceptedAnswer' => array(
                                '@type' => "Answer",
                                'text' => get_sub_field('answer')
                                )
                                );
                                array_push($schema['mainEntity'], $questions);
                                        endwhile;
                    function generate_faq_schema ($schema) {
                    global $schema;
                    echo '<script type="application/ld+json">'. json_encode($schema) .'</script>';
                    }
                    add_action( 'wp_footer', 'generate_faq_schema', 100 );
                    }
                    ?>
                    <?php endif; ?>
                    <!-- endif have_rows('faq'); -->
                </div>
            </div>
        </div>
    </div>
</section>

After lots of testing, both answers received some sort of a result, but the actually working code has no errors and is pulling through to Google is above.

Please feel free to make any suggestion on making this better.