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));