You cannot post a purchase order line item to Accounts payable because that is the account credited by a purchase invoice. The accounts you select in the dropdown list for each line item is the account debited.
See these Guides:
For invoices that predate your start date, you could combine all invoices for one supplier, as a kind of pseudo-invoice. But you will still need to enter every line item and could lose the ability to resolve issues with suppliers if there are problems. I would certainly not recommend it. The point of creating the purchase invoices is not just to establish the monetary amount owed, but also to track/modify incoming inventory items.
That is the joy of owning a business. Of course, if your business was ongoing before adopting Manager, you might consider paying off those purchase invoices first, before making the transition to Manager. But perhaps there is never a good time. On the other hand, if you are just starting your business, and had no prior system, you shouldn’t be setting a start date at all. You should just enter all transactions relevant to the business from its inception.