необходимо добавить псевдонимы, иначе тренажер не примет

SELECT i.billing_country,
       MIN(i.total) AS min_total,
       MAX(i.total) AS max_total,
       AVG(i.total) AS avg_total
FROM invoice AS i
WHERE i.invoice_id IN
    (SELECT invoice_id
     FROM invoice_line
     GROUP BY invoice_id
     HAVING COUNT(invoice_line_id) > 5)
  AND i.total >
    (SELECT AVG(unit_price)
     FROM invoice_line)
GROUP BY i.billing_country
ORDER BY avg_total DESC, i.billing_country;