One line answer - Sub-Queries are not allowed.
But if you need to handle your scenario, then you can do the following:
Encapsulate your entire query into a stored procedure and then use the stored procedure.
Create a view. The view is created to handle either your main query or sub-query.
Create a table level variable and store the end results of view in it and then go ahead and fetch the value of table variable in the main query.