Just like sales invoices, before you start processing purchase invoices, you must verify the Sales Taxes are setup properly. Importantly, if you setup the sales taxes incorrectly, the transaction will remember the incorrect Sales Tax setup and you will need to Reverse / Void the transaction entirely in order to apply the new proper settings to the already posted invoices
After you setup the sales taxes, you will want to verify in a test transaction that it is aligned with the correct GL accounts by or going to CTRL + J while in the Purchase Invoice
If you entered a GL beginning balance total for Accounts Payable when working with the Chart of Accounts, then you should have outstanding, unpaid customer invoices