When a donor submits a recurring donation using the Soapbox Engage Donations app pages, an appropriate Recurring Donation record should be made in the NPSP. If one isn't made, here's what could be causing it not to be created.
1) The Default Contact Role for "Contact Role for Individual Opps" is not set
Make sure that you have selected a contact role in Salesforce > Nonprofit Success Pack Settings > Donations > Contact Roles > Default Contact Roles > Contact Role for Individual Opps. If this is set to "--None--", the Soapbox Donations for Salesforce package won't fire correctly, and will not properly create the Recurring Donation record.