If you want to add HSN Code to a new item then go to Sales -> Add New Stock Item -> Now fill the appropriate details after that click on the GST Adv Tax Settings and add the HSN Code in the column.
Or if you want to add HSN Code to an existing item then go to Sales -> Edit/Delete Stock Item -> Now enter the item id or item name and click Edit -> A new window will open and click Edit -> Click on the GST Adv Tax Settings and add the HSN Code in the column.