Access Plugin data from Functions.php

Your syntax is slightly off in a few places. I’ll try and let the code explain:

function wpse_199498_meta_to_table( $post_id, $post ) {
    // Use the second argument of our hook, which is the post object
    if ( $post->post_type !== 'festival-event' )
        return;

    global $wpdb;

    // Don't need to use $_POST, just use the functions as documented
    $custom_lat = get_geocode_lat( $post_id );
    $custom_lng = get_geocode_lng( $post_id );

    $check_link = $wpdb->get_var(
        $wpdb->prepare(
            "SELECT post_id FROM lat_lng_post WHERE post_id = %d", // Here's that missing comma!
            $post_id
        )
    );

    if ( $check_link ) {
        $wpdb->update(
            'lang_lng_post',
            array(   
                'lat' => $custom_lat,
                'lng' => $custom_lng,
            ),   
            array(
                'post_id' => $post_id,
            ),   
            array(   
                '%f',  
                '%f',
            ),
            '%d'
        );
    } else {
        $wpdb->insert(
            'lang_lng_post',
            array(   
                'post_id' => $post_id,
                'lat'     => $custom_lat,
                'lng'     => $custom_lng,
            ),   
            array(  
                '%d',
                '%f',  
                '%f',
            )
        );
    }
}

add_action(
    'wp_insert_post',            // Later than "save_post", will ensure plugin has already saved meta at this point
    'wpse_199498_meta_to_table',
    10,                          // Default priority
    2                            // Number of arguments
);