When you push a purchase order from Inventory Planner to Quickbooks Online (QBO), Inventory Planner creates an identical PO with the same products, quantities, cost that you see on the PO dashboard side.
Before saving a PO you need to make sure that all products are created in QBO. Otherwise you will get an error - Inventory Planner doesn't create products in QBO automatically.
Note that this integration works with Quickbooks Online, but not Quickbooks Desktop or Quickbooks Enterprise.
Go to Account > Settings
> Purchase Orders and you can enable saving POs to Quickbooks.
Then after you've created a new purchase order, you'll see the option to Save to Quickbooks.