Original code:
-----------------------------------------------------------
$output       .= '<span class="post-updated"> ' . $modified_on . '</span>';



Corrected code:
-----------------------------------------------------------
$output       .= '<span class="post-updated" itemprop="dateModified"> ' . $modified_on . '</span>';




Complete code:
-----------------------------------------------------------
/**
 * Display only last modified date in the post metadata.
 *
 * @param String $output Markup for the last modified date.
 * @return void
 */
function your_prefix_post_date( $output ) {
    $output        = '';
    $format        = apply_filters( 'astra_post_date_format', '' );
    $modified_date = esc_html( get_the_modified_date( $format ) );
    $modified_on   = sprintf(
        esc_html( '%s' ),
        $modified_date
    );
    $output       .= '<span class="posted-on">';
    $output       .= '<span class="post-updated" itemprop="dateModified"> ' . $modified_on . '</span>';
    $output       .= '</span>';
    return $output;
}
add_filter( 'astra_post_date', 'your_prefix_post_date' );