You Can add This below Snippet code for :

change your input value 'true' and add after <?php echo $sidebar?'checked': 'unchecked'; ?> in input element

function theme_post_sidebar_settings($post) {

    $sidebar = get_post_meta($post->ID, '_theme_post_meta_sidebar', true);
    wp_nonce_field( 'theme_update_post_sidebar_settings', 'theme_update_post_sidebar_nonce' );
    ?>

    <input type="checkbox" name="theme_post_meta_sidebar_field" id="theme_post_meta_sidebar_field" value="true" <?php  echo $sidebar?'checked': 'unchecked'; ?> />
    <label for="theme_post_meta_sidebar_field"><?php esc_html_e( 'Sidebar', 'theme' ); ?></label>

    <?php
}

You Can add This below Snippet code for :

change your input value 'true' and add after <?php echo $sidebar?'checked': 'unchecked'; ?> in input element

function theme_post_sidebar_settings($post) {

    $sidebar = get_post_meta($post->ID, '_theme_post_meta_sidebar', true);
    wp_nonce_field( 'theme_update_post_sidebar_settings', 'theme_update_post_sidebar_nonce' );
    ?>

    <input type="checkbox" name="theme_post_meta_sidebar_field" id="theme_post_meta_sidebar_field" value="true" <?php  echo $sidebar?'checked': 'unchecked'; ?> />
    <label for="theme_post_meta_sidebar_field"><?php esc_html_e( 'Sidebar', 'theme' ); ?></label>

    <?php
}

Change this below function

function theme_save_post_sidebar_settings($post_id, $post) {

        $edit_cap = get_post_type_object( $post->post_type )->cap->edit_post;
        if( !current_user_can( $edit_cap, $post_id )) {
            return;
        }
        if( !isset( $_POST['theme_update_post_sidebar_nonce']) || !wp_verify_nonce( $_POST['theme_update_post_sidebar_nonce'], 'theme_update_post_sidebar_settings' )) {
            return;
        }
        if(defined('DOING_AUTOSAVE') && DOING_AUTOSAVE) {
            return;
        }

            update_post_meta( 
                $post_id, 
                '_theme_post_meta_sidebar', 
                sanitize_text_field($_POST['theme_post_meta_sidebar_field'])
            );
        
    }
    add_action( 'save_post', 'theme_save_post_sidebar_settings', 10, 2 );

You Can add This below Snippet code for :

change your input value 'true' and add after <?php echo $sidebar?'checked': 'unchecked'; ?>

function theme_post_sidebar_settings($post) {

    $sidebar = get_post_meta($post->ID, '_theme_post_meta_sidebar', true);
    wp_nonce_field( 'theme_update_post_sidebar_settings', 'theme_update_post_sidebar_nonce' );
    ?>

    <input type="checkbox" name="theme_post_meta_sidebar_field" id="theme_post_meta_sidebar_field" value="true" <?php  echo $sidebar?'checked': 'unchecked'; ?> />
    <label for="theme_post_meta_sidebar_field"><?php esc_html_e( 'Sidebar', 'theme' ); ?></label>

    <?php
}

You Can add This below Snippet code for :

change your input value 'true' and add after <?php echo $sidebar?'checked': 'unchecked'; ?> in input element

function theme_post_sidebar_settings($post) {

    $sidebar = get_post_meta($post->ID, '_theme_post_meta_sidebar', true);
    wp_nonce_field( 'theme_update_post_sidebar_settings', 'theme_update_post_sidebar_nonce' );
    ?>

    <input type="checkbox" name="theme_post_meta_sidebar_field" id="theme_post_meta_sidebar_field" value="true" <?php  echo $sidebar?'checked': 'unchecked'; ?> />
    <label for="theme_post_meta_sidebar_field"><?php esc_html_e( 'Sidebar', 'theme' ); ?></label>

    <?php
}