COUNT(p.post_id) AS total_posts
を含めないでください SELECT
で サブクエリにリストします。ちょうど
SELECT topic_id
FROM phpbb_topics AS t
WHERE t.topic_id IN (
SELECT p.topic_id --, COUNT(p.post_id) AS total_posts
FROM phpbb_posts AS p
WHERE p.poster_id = 61640
GROUP BY p.topic_id
HAVING t.topic_replies_real = COUNT(p.post_id) - 1
);