To initiate a scheduled invoice generation, an Autolaunched Flow invoking the Invoice API invocable class should be created where the flow will be called in a Batch Job.
...
Go to Setup.
In the Quick Find box, search and select Batch Management.
Click New.
Enter some required data and click the autolaunch flow that’s been created a while ago and click Next. In next screen, select the recordId as Flow Input Variable and Opportunity as Object
Note: The required batch size would be 60 only to prevent hitting the governor limit
...
Click Save.
Create a Schedule Flow
...
Drag an action and find the batch name that you created a while ago in batch management and select it.
7. Click Save and activate.
Scheduling and Running a Batch Apex Class to Link Orphan Gateway Transaction Records to Gateway Invoice Records Using the Developer Console
In the home page, click the settings and click developer settings
...
In the Developer Console page, click the debug and click open Execute Anonymous Window
...
Paste this code (Note: you can change the schedule name)
System.schedule('Schedule_OrphanGatewayTransactionInvoiceLinker 1', '0 30 * * * ?', new PosiPay__OrphanGatewayTransactionInvoiceLinker());
...
Next place to go : Step 6 - Schedule a Gateway Settlement
...