When creating a Purchase Order, what does 'select an account for this transaction' mean?

Hi there, @david355.

A purchase order in QuickBooks Online help keep track of the items you've ordered; they have no bearing on any of your accounts. 

The error you're getting "You must select an account for this transaction" usually pop up when there's an added extra line in the items list that don't have a Product/Service but description, amount, etc.

To fix this, you'll want to create a new Product/Service for the line you're trying to add, and used that instead. Here's how to add a service or product’s purchasing info:

1. Go to the Sales menu, then select Products & services.
2. Find the product or service on the list, then select Edit in the Action column.
3. Choose I purchase this product/service from a vendor, then add a description to show on bills, purchase orders, and other forms you send to vendors.
4. Add the Cost, then select the Expense account in the dropdown.
5. Choose the account you use to track the cost of things you sell.
6. Select your preferred vendor.
7. Click Save and close.

You can check out this article to learn more about adding product and service items to QuickBooks Online.

Get back and post again you need helping hands in managing your purchase orders and other transactions. Take care!