the_excerpt() does not work with has_excerpt()?

excerpt_length is only applied to generated excerpts— excerpts automatically generated from the post body.

To trim your manually created excerpt, use the same function that trims the generated one— wp_trim_words.

if(has_excerpt()) { 
  $length = apply_filters('excerpt_length',20);
  echo wp_trim_words(get_the_excerpt(),$length); 
} else { 
  the_content(); 
}