SQL Server throws an exception (RAISERROR)

Create PROCEDURE [dbo]. [Sptasks_assigned_update]    (      @ID uniqueidentifier OUTPUT,      @MODIFIED_USER_ID uniqueidentifier = NULL,      @ASSIGNED_USER_ID uniqueidentifier = NULL                 ) as    DECLARE @ASSIGNED_USER_NAME NVARCHAR (+);    SELECT  @ASSIGNED_USER_NAME = Assigned_to_name    from    dbo.vwtasks    WHERE   ID = @ID;    IF @ASSIGNED_USER_NAME is NULL        BEGIN            UPDATE  [TASKS]            SET     [modified_user_id] = @MODIFIED_ USER_ID,                    [date_modified] = GETDATE (),                    [assigned_user_id] = @ASSIGNED_USER_ID,                    STATUS = ' In progress ' C21/>where   [ID] = @ID;           END;    ELSE        BEGIN            RAISERROR        (N ' Submit failed, this task was assigned to%s! ', +      ,--Severity.      1,--state.    @ASSIGNED_USER_NAME  );              END;                                             

　　

SQL Server throws an exception (RAISERROR)