$query = "INSERT INTO products_configurations (product_id, cuts)
VALUES (SELECT pro.id FROM pages AS p
JOIN products AS pro ON pro.page_id = p.id WHERE p.id = $pageid, $combination)";

$pageid = 21605
$combination = "40 - Red"

You have an error in your SQL syntax;
check the manual that corresponds to your MySQL server version
for the right syntax to use near
'SELECT pro.id FROM pages AS p
JOIN products AS pro ON pro.page_id = p.id WHERE p' at line 1