I figured it out! This is what worked:

@Body2 =  ISNULL(@Body2,'') + 'HTML code'

What do you mean by "have not initialised your variables"?

Also, there are no NULL values in the data currently.

Here is the full code:

DECLARE @recipientsX varchar(150)
    DECLARE @copy_recipientsX varchar(150)
    DECLARE @blind_copy_recipientsX varchar(150)
    DECLARE @SubjectLineX varchar(150)
    DECLARE @attachmX1 varchar(100)
    DECLARE @attachmX2 varchar(100)
    DECLARE @header varchar(MAX)    

I got it to work in SQL but I figured out that it's not a SQL issue this line works:

select * Into NewTabe from( select *, row_number()over(partition by Code order...

Can you please format it into something more relevant/shorter? I am very new to SQL and do not quite understand your syntax, please see my code.