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.