Row_Num Partition in WHERE clause
From Itzik Ben-Gan's Microsoft SQL Server 2012 High-Performance T-SQL Using Window Functions (Developer Reference):

Logical Query Processing Order:


FROM
WHERE
GROUP BY
HAVING
SELECT


Evaluate Expressions  
Remove Duplicates  

ORDER BY
OFFSET-FETCH/TOP


Window Functions operate in steps 5.1. and 6. 

Obviously then, you cannot access the results of a Window Function in the WHERE or HAVING clauses. You can always use a CTE or a subquery.


See also:
- Logical Processing Order of the SELECT statement (SQL Server 2014 - 2017)
- Logical Query Processing: What It Is And What It Means to You (Itzik Ben Gavin - ITProToday)