Si intenta ACTUALIZAR una columna directamente usando ROW_NUMBER() obtendrá...
...así que en su lugar INNER JOIN la tabla a sí misma...
UPDATE
[test123]
SET
[row_number] = [x].[rn]
FROM
[test123]
INNER JOIN
(
SELECT
[test_id],
ROW_NUMBER() OVER (ORDER BY [test_id]) AS rn
FROM
[test123]
) AS x
ON
[test123].[test_id] = [x].[test_id]