Which edition of QB do i need to add a purchase ledger ?

You need to use QBO Plus or Advanced to have the purchase order feature.

Another option, consider QB Desktop 2024 Pro or Premier.

Welcome to the Community, toft. 

As mentioned by @4Gal, Only QuickBooks Online's Plus and Advanced, as well as Desktop's 2024 Pro and Premier, have the purchase order feature.

If you'd like to access this feature, let me help you upgrade your QBO subscription. Here's how: 

1. Choose a plan that is right for you. 
2. Sign in to your QBO account as a primary or company admin. 
3. Go to the Gear icon and choose Subscription and Billing. 
4. Ensure that your payment information is up to date.
5. In the QuickBooks Online section, select the Upgrade your plan.
6. Review the available plans and click the Choose plan. 
7. Follow the on-screen steps on how you can upgrade your plan.

You can refer to this article to learn how to record a purchase order and record it as a bill or expense: Add purchase orders to expenses, bills, or checks in QuickBooks Online.

Please let us know if you have other questions regarding subscriptions and purchase order in QBO. Here in Community space, we're always ready to help you.