ISNULL takes two parameters:
SQL Server ISNULL() Function[
^]
It works for me:
USE [Testing]
GO
SET ANSI_NULLS ON
GO
SET QUOTED_IDENTIFIER ON
GO
CREATE TABLE [dbo].[MyTable](
[ID] [int] NOT NULL,
[Text] [nchar](3) NOT NULL,
[SerialNo] AS (isnull([Text]+right('00000'+CONVERT([varchar](5),[ID],(0)),(5)),'????????')),
CONSTRAINT [PK_MyTable] PRIMARY KEY CLUSTERED
(
[ID] ASC
)WITH (PAD_INDEX = OFF, STATISTICS_NORECOMPUTE = OFF, IGNORE_DUP_KEY = OFF, ALLOW_ROW_LOCKS = ON, ALLOW_PAGE_LOCKS = ON) ON [PRIMARY]
) ON [PRIMARY]
GO