Cannot find either column "dbo" or the user-defined function or aggregate "dbo.fnRegistrationNoExp", or the name is ambiguous.

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Here, I am generate WardNo+6 digit = Registration No...How can i test the funtion ....error in Bold Capital words please .....give solution...
I'm getting a frustrating error in one of my SQL Server 2008 r2 queries. It parses fine, but crashes when I try to execute. The error I get is the following :

Msg 8120, Level 16, State 1, Line 1
Column 'dbo.FamilyHead.WardID' is invalid in the select list because it is not contained in either an aggregate function or the GROUP BY clause.

fnFormatSerialNoPostfix:- it's function genrate 6 digite no it's...ok

SET @RegistrationNo=(SELECT CONVERT(Varchar(15),CONVERT(varchar,@WardID) + 'W-') +         CONVERT(varchar(6),dbo.fnFormatSerialNoPostfix(CONVERT(int,SUBSTRING(MAX(RegistrationNo),10,6)+1))) FROM dbo.ParivarCertificate INNER JOIN dbo.FamilyMember ON dbo.ParivarCertificate.FamilyMemberID = dbo.FamilyMember.FamilyMemberID  INNER JOIN dbo.FamilyHead ON dbo.FamilyMember.FamilyID = dbo.FamilyHead.FamilyID WHERE dbo.FamilyHead.WardID = @WardID )

How to fix it.... All Code bellow...

ALTER FUNCTION [dbo].[fnRegitrationNoExp] (@WardID int)

RETURNS Varchar(12)
AS
BEGIN
    -- Declare the return variable here
        DECLARE @RegistrationNo Varchar(15)
        if Exists(SELECT RegistrationNo FROM dbo.ParivarCertificate
                  INNER JOIN dbo.FamilyMember ON dbo.ParivarCertificate.FamilyMemberID = dbo.FamilyMember.FamilyMemberID
                  INNER JOIN dbo.FamilyHead ON dbo.FamilyMember.FamilyID = dbo.FamilyHead.FamilyID
                  WHERE dbo.FamilyHead.WardID =@WardID)
                  --WHERE FH.WardID=@Ward)

            Begin

                SET @RegistrationNo=(SELECT CONVERT(Varchar(15),CONVERT(varchar,@WardID) + 'W-') +
                CONVERT(varchar(6),dbo.fnFormatSerialNoPostfix(CONVERT(int,SUBSTRING(MAX(RegistrationNo),10,6)+1)))
                FROM dbo.ParivarCertificate
                  INNER JOIN dbo.FamilyMember ON dbo.ParivarCertificate.FamilyMemberID = dbo.FamilyMember.FamilyMemberID
                  INNER JOIN dbo.FamilyHead ON dbo.FamilyMember.FamilyID = dbo.FamilyHead.FamilyID
                  WHERE dbo.FamilyHead.WardID = @WardID )

            End
        Else
        Begin

                SET @RegistrationNo=(SELECT CONVERT(varchar(15),CONVERT(int,@WardID) + 'W-') +
                CONVERT(varchar(6),dbo.fnFormatSerialNoPostfix(1)))

        End

        -- Return the result of the function
        RETURN @RegistrationNo

END