I can reproduce the same error message myself when writing a similar query. My understanding is that you are **missing a projection** on top of the query.
You can easily sort this by adding an extra select statement. As an example, have a look at this VQL:
CREATE OR REPLACE VIEW iv_customer_address
DESCRIPTION = '...' (text1 (DESCRIPTION = '…'))
SELECT '1' as text1, '2' as text2, '3' as text3 FROM
(SELECT '1' as text1, '2' as text2, '3' as text3
(SELECT '1', '2', '3'
FROM address a
INNER JOIN customer b
ON a.address_id = b.address_id))
There is **one recommendation**: Denodo Platform doesn’t let you edit the query if you write it using two statements at once, as it does not know how to put it graphically. Therefore, to avoid these errors, you should perform these statements separately and union these graphically as the Denodo Platform easily allows you to.
Hope this helps!