Попробуйте это -
ALTER PROCEDURE [dbo].[prViewRequirements]
@WFRouteID INT
, @DocumentDescription VARCHAR(100) OUTPUT
AS BEGIN
SELECT @DocumentDescription = t.DocumentDescription
FROM dbo.tbFollowOnTracking t
WHERE t.WFRouteID = @WFRouteID
AND t.IsActive = 1
IF @DocumentDescription IS NULL
RETURN -1
RETURN 0
END