Just copy below Function and Add a Function in your Divi child theme function.php file.

<?php

add_filter( 'wpseo_metadesc', 'sm_divi_builder_content_wpseo_metadesc', 10, 1 );
function sm_divi_builder_content_wpseo_metadesc( $desc ) {
 // Get $post if you're inside a function
 global $post;
 $post_type = get_post_type( $post );
 if ($post_type == "page") {
  //get_meta_description
  $desc = get_post_meta($post->ID, '_yoast_wpseo_metadesc', true); 
  if($desc=="")
  {
   if ( has_excerpt( $post->ID ) ) {
    // This post has excerpt
    $desc = $post->post_excerpt;
   } else {
    // This post has no excerpt
    $desc = trim(strip_tags(preg_replace('/\[\/?et_pb.*?\]/', '', $post->post_content)));
    $desc = substr($desc,0,155); 
   }
  }
 }
 return $desc;
}

?>