Postgres giving relation does not exist error for alias

Question

WITH TAB AS 
(
 SELECT id,flag FROM source where flag IS NULL and Data= '151234567890' ORDER BY 1 OFFSET 
 (
  SELECT COUNT(*) FROM source where flag IS NULL and Data = '151234567890' ) - 96 FETCH FIRST 96 ROWS ONLY )
 UPDATE TAB SET flag = 'Z';

Above query is working fine in MS Sql but showing this error when running it in postgres

ERROR:  relation "tab" does not exist
LINE 9:  UPDATE TAB SET flag = 'Z';