org.hibernate.QueryException: illegal attempt to dereference collection

Question

billProductSet is a Collection.
As such, it does not have an attribute named product.

Product is an attribute of the elements of this Collection.

You can fix the issue by joining the collection instead of dereferencing it:

SELECT count(*) 
  FROM BillDetails        bd 
  JOIN bd.billProductSet  bps 
 WHERE bd.client.id       = 1
   AND bps.product.id     = 1002