Hello,
I will guide you step by step in order to allow to check that you don't forget a detail :
1°) Custom field settings, take a good care of red bordered setting on my screenshot
In Main HikaSHop Configuration => Display dropdown select Custom field, then create the New Custom field
2°) See the result in your order listing => Order Number to
edit your order :
3°) Result in the Email, from
HikaShop Main Configuration, go in
System dropdown select
Emails, and edit the
Order status notification, click on
Preview button, and select a order (bu his number) processed
AFTER you have made previous settings, see my screenshot :
Hope this will help you to get what is lacking.
Regards