SQL ERROR in my WHERE CLAUSE QUERY

SQL ERROR in my WHERE CLAUSE QUERY

SELECT product_key.contact_email, product_key.client_name,
product_key.status, product_key.key, payment.paymentdate, product_key.id,
MAX(paymentdate) AS latest_payment, DATE_ADD(MAX(paymentdate), INTERVAL 1
MONTH) AS expiration_date
FROM product_key LEFT OUTER JOIN payment ON payment.keyid=product_key.id
WHERE product_key.status = 'purchased' AND expiration_date =
DATE_ADD(NOW(), INTERVAL 10 DAY) GROUP BY product_key.id
ORDER BY client_name asc
This is my query. I know I can't use an alias in a WHERE Clause since
WHERE is first read before the SELECT. But even if I use something like
this:
SELECT product_key.client_name, DATE_ADD(MAX(paymentdate), INTERVAL 1
MONTH) AS expiration_date
FROM product_key LEFT OUTER JOIN payment ON payment.keyid=product_key.id
WHERE DATE_ADD(MAX(paymentdate), INTERVAL 1 MONTH) = DATE_ADD(NOW(),
INTERVAL 10 DAY) AND product_key.status = 'purchased'
GROUP BY product_key.id
ORDER BY client_name asc
Still an error. Please help. Thanks.