How can I create a monthly report showing expense items which were not paid during the month and items paid but expensed in a prior month?

Hello there, @jonathanmogil-fe.

Let's pull up the Transaction List by Vendor and Check Detail reports to show paid and unpaid expense items in QuickBooks Online (QBO). 

You can run and customize the Transaction List by Vendor report, so you can view your expense items that were not paid for a specific period (during the month). Then, pull up the Check Detail report for the ones paid and expensed for a certain month (prior). Please make sure to set the appropriate date to get the data you need. To do this, here's how:

1. Go to Business overview, then select Reports, or go to Reports.
2. Use the Find report by name search bar to search for reports by name. Pull up either the Transaction List by Vendor or the Check Detail report.
3. Click the report to open it.
4. Select Customize button to focus on the details you need. Set the appropriate date and all other data you need.
5. Click Run report.

Once you're done, you can memorize the reports to save their current customization settings.

Also, you may want to check out this article as your reference to guide you in learning specific ways to customize reports to fit your specific needs: Common custom reports in QuickBooks Online.

Let me know if you have other reporting concerns or questions about managing expense transactions in QBO. I'll gladly help. Take care, and have a great day, @jonathanmogil-fe.

Thank you for the reply.  I tried both those reports but unfortunately they don't solve the issue I have.  The Transaction List by Vendor report shows all the transactions, not just the ones which have not been paid.  The Check Detail Report also lists all the payments, not just the ones where the expense occurred in an earlier month.  I have tried various filters but still no luck. 

I know how this type of report is useful for your business, jonathanmogil-fe.

You'll want to go to the Customer Feedback page to let our product team know the importance of the such report. To do this, you can click on the Give Feedback option. I've added this screenshot for your guide:
 

Aside from customizing your report so you'll get the information you need, you can export them to individual Excel files contained in a zip format in one just one process. To learn more on how to do this, I've got this article for your guide: Export Reports, List, and Other Data From QuickBooks Online.
 

Got more questions with reports. Well, drop your comment below and I'll make sure to get back.