You can enter the quantity (QTY) and price detail or amount on the Item Details section.
That's for items from products and services list like the purchase of inventory items by quantity and unit price. Whereas the Category Details refers to a selection of an account from the company chart of accounts to categorize expense type (or non-expense type).
Go to the + New button and choose Bill.
Fill out all the necessary fields.
Provide the needed information like the amount and quantity under the QTY and Amount column.
For more information about managing Accounts Payable transactions, see this article: Enter and pay bills.
If you have other questions or need further assistance in creating a bill, let me know in the reply section. I'll be more than happy to help. Have a wonderful day!