If you activated the automated invoice cancellation you may want to check first which invoices would get cancelled.
The module does have a test mode integrated. To run a test, open an invoice at the WHMCS Admin, click on View as Client and add &testAutocancellations=1 at the end of the URL.
The testmode link looks like this: whmcs.yourdomain.com/viewinvoice.php?id=12&view_as_client=1&testAutocancellations=1
After calling the URL, the cancelled invoices are logged to your WHMCS Activity Log Utilities → Logs → Activity Log.
You need to be logged in as admin, otherwise, the test mode does not execute.