When your notifications screen has the message which says, You have AP Open Items that are past their Due Date, this means there are unpaid invoices which have been posted to Select Items for Payment whose due-date has past.
You may find these invoices in one of two ways:
First, if you click on the Notifications Link, it opens Select Items for Payment.
In the Select Checking Account Window, select each account you have on file one at a time.
One of the accounts is likely to show an unpaid item.
If you determine this invoice is no longer necessary:
If you are on a Cash Basis of Accounting (File, Preferences, Accounts Payable, 'Use Cash instead of Accrual' is marked, you may simply reverse this item by clicking into the Type and changing it to 'R' for Reverse, and then by selecting the Post Already Paid/Reverse button at the bottom left of the screen.
When you click the button, click ok for the following pop-up:
Since this is a cash basis system, the Posting Date does not matter because the invoice is just going to be deleted and will not affect your books. Simply leave the next screen as it is and click Post. NOTE: Do not mark the Restore to AP Transactions box.
If you are using an Accrual Basis of Accounting, follow all of the steps above except the last step. You must use a Posting date in an Open Accounting Period (Year).
This is because when you create an invoice in an Accrual Basis Accounting System, the AP Account is Credited on the Invoice Date and the Expenses are Debited on the Invoice Date. When you delete the invoice, a reversing entry will be created to reverse the Accounts Payable and Expenses. If you Post that entry as of a previous year, this will throw your Year-End Balances out and you will also have to reprint all of your year-end reports. If the date is several years ago, you would have to open the year, post the entry and then close that year and each subsequent year. All previously printed statements will be affected.
The other option is to print your Open Items Analysis, Detailed Aging Report to find any transactions that are past due.