CREATE FUNCTION failed because a column name is not specified for column 1.... in Functions sql server 2008

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I am use the function for generate Uniq No WardID+SerialNo
WardID Comes from table FamilyHead
SerialNo Save in FamilyMember

WardID     Uniq No Should be Generated
12         12000001,12000002,12000003 and so on..
13         13000001,13000002,13000003 and so on...
15         15000001,15000002,15000003 and so on....

I use this type logic it's not work properly
can you help please....

ALTER function [dbo].[fnFamilyUniqueID](@WardID int) returns table
as
RETURN
(

SELECT CONVERT(Varchar(15),CONVERT(varchar,@WardID) + 'W-') +
       CONVERT(varchar(6),dbo.fnFormatSerialNoPostfix

       (CONVERT(int,SUBSTRING(MAX(SerialNo),10,6)+1))
       FROM FamilyMember
       inner join FamilyHead on FamilyMember.FamilyID = FamilyHead.FamilyID
       WHERE FamilyHead.WardID=@WardID
}

Where dbo.fnFormatSerialNoPostfix is... generate the 6 digit no

ALTER Function [dbo].[fnFormatSerialNoPostfix]
(@SerialNo Int)
Returns varchar(10)
As
Begin

      return Right(Replicate('0',6)+Convert(Varchar(10),@SerialNo),6)

End