I can add more information about setting up sub-items in QuickBooks Desktop, All Brands Group.
The warning You must specify an account to be associated with this item. will pop-up if you click the OK tab without assigning an account for the item. Just go to the Account field and select the appropriate information to successfully save it.
If you wish that the Parker Refill and Parker Pen will be under the Parker item, just edit the items and put a check mark on the Subitem of box. Then, select Parker as the main item.
This is how the Item List will show:
My colleague already provided this link, but I'll add it again so you don't have to search for it: Add, edit, and delete items.
Please let me know how it goes or if you have any follow-up questions about items. I'm still here to offer more help.