How to add together (get the sum) a field from all author’s posts

First you have to declare the global $wpdb before using it. Also you need to include the table prefix when calling the table names.

global $wpdb;
$posts_table = $wpdb->prefix . "posts";
$posts_meta_table = $wpdb->prefix . "postmeta";

Next get an array of post ids by the author needed( make sure you have the id of desired author, assign to $author_id).

$authors_posts = $wpdb->get_col($wpdb->prepare("SELECT * FROM ".$posts_table." WHERE post_author = %d", $author_id) );

Make a comma delimited version of the post id array for SQL

$comma_delimited = implode(',', array_map('intval', $author_posts));

Next use the id array to make a sum of ‘epicredvote’ from the post meta table. Note the # of votes is stored in the meta_value column

$allscore = $wpdb->get_var( $wpdb->prepare(
    "SELECT SUM(meta_value) 
    FROM " .$posts_meta_table . "
    WHERE (meta_key = 'epicredvote' 
           AND post_id IN %s)"
   , $comma_delimited));