Force Sidebar on Full Width page

I’m not certain this would work without seeing the theme but you could try something like this…

Change…

if ( $sidebar_layout === 'sidebar-left' && empty( $zerif_change_to_full_width ) ) {
    zerif_sidebar_trigger();
}

To this…

if ( $sidebar_layout === 'sidebar-left' && empty( $zerif_change_to_full_width ) || is_singular('post')) {
    zerif_sidebar_trigger();
}

This, in theory, would force the left sidebar on posts because I added || is_singular('post'), which simply means OR is a post.

You could do the same thing for where its checking for the right sidebar.

if ( $sidebar_layout === 'sidebar-right' && empty( $zerif_change_to_full_width ) || is_singular('post')) {
     zerif_sidebar_trigger();
}

Since we changed our conditions around we also need to add in some logic to handle the class that is being used.

Change…

if ( ! empty( $zerif_change_to_full_width ) || 'full-width' === $wrapper_class ) {
    echo '<div class="content-left-wrap col-md-12">';
} else {
    echo '<div class="content-left-wrap col-md-9">';
}

To…

if(is_singular('post')){
    echo '<div class="content-left-wrap col-md-9">';
}elseif ( ! empty( $zerif_change_to_full_width ) || 'full-width' === $wrapper_class ) {
    echo '<div class="content-left-wrap col-md-12">';
} else {
    echo '<div class="content-left-wrap col-md-9">';
}

Update: Here is what your full code would look like with the right sidebar. Noticed I moved || is_singular('post') down to the last if statement.

<?php

/**
 * The Template for displaying all single posts.
 *
 * @package zerif
 */

get_header();

$sidebar_layout = apply_filters('zerif_sidebar_layout', get_theme_mod('zerif_blog_sidebar_layout', 'sidebar-right'));
$wrapper_class="content-left-wrap col-md-9";

if ('full-width' === $sidebar_layout) {
    $wrapper_class="content-left-wrap col-md-12";
}

?>
<div class="clear"></div>

</header> <!-- / END HOME SECTION  -->

<?php

zerif_after_header_trigger();

if (apply_filters('zerif_display_area_filter', true, 'single')) {
    $zerif_change_to_full_width = get_theme_mod('zerif_change_to_full_width');
?>

<div id="content" class="site-content">
        <div class="container">
            <?php
    zerif_before_single_post_trigger();

    if ($sidebar_layout === 'sidebar-right' && empty($zerif_change_to_full_width)) {
        zerif_sidebar_trigger();
    }
    if (is_singular('post')) {
        echo '<div class="content-left-wrap col-md-9">';
    } elseif (!empty($zerif_change_to_full_width) || 'full-width' === $wrapper_class) {
        echo '<div class="content-left-wrap col-md-12">';
    } else {
        echo '<div class="content-left-wrap col-md-9">';
    }

    ?>
    <?php
        zerif_top_single_post_trigger();
    ?>
        <div id="primary" class="content-area">
                <main itemscope itemtype="http://schema.org/WebPageElement" itemprop="mainContentOfPage" id="main" class="site-main">
    <?php
        while (have_posts()) {
            the_post();
            get_template_part('content', 'single');
            zerif_post_nav();
            /* If comments are open or we have at least one comment, load up the comment template */
            if (comments_open() || '0' != get_comments_number()) {
                comments_template('');
            }
        }
    ?>
            </main><!-- #main -->
            </div><!-- #primary -->
    <?php
        zerif_bottom_single_post_trigger();
    ?>
    </div><!-- .content-left-wrap -->
    <?php
        zerif_after_single_post_trigger();

        if ($sidebar_layout === 'sidebar-right' && empty($zerif_change_to_full_width) || is_singular('post')) {
            zerif_sidebar_trigger();
        }
    ?>
</div><!-- .container -->
    </div>
    <?php
}
get_footer();
?>