Based on your formula, you are checking what weeks are connected with the 1st year forecast (IF 'SYS01 Time Settings By Week'.'1st Forecast Year?') and giving them some values (THEN 'DAT03 Historic Volumes'.'Offset Volumes for 1st Forecast Year').
As you can see, you are defining what is out of the scope as 0 (ELSE 0), which means that you need to connect the data from the "Default Forecast" in the previous year with your next year (LAG or OFFSET).