If you need to create recurring invoices (sales or purchase) then you can by going to

From here you will see

Note, the bottom part of the above screen shows the events (invoices that will be created) that will take place because of the recurring event

Pressing 'New' will show

Now you can set the recurrence details for the Recurring Event.  You should give it a meaningful name so it can be easily found if you need to cancel or edit it.

The event uses Invoice Templates to determine what to add to the new invoice.