Chyba je celkem jasná. Každá odvozená tabulka musí mít svůj vlastní alias. Musíte zadat alias (SELECT @rownum := 0)
takhle:
SELECT
@rownum := @rownum + 1 AS rank,
name,
vote
FROM table, (SELECT @rownum := 0) t --This what you were missing an alias
ORDER BY vote DESC