Why do donors see the error "...List has no rows for assignment to SObject (sbxd)" on the confirmation page?

If your visitors are seeing the following error message when submitting a Donations app page...

"Salesforce Error: sbxd.sbx_DonationsRecurringPaymentLead: execution of AfterInsert caused by: System.QueryException: List has no rows for assignment to SObject (sbxd)"

...it's likely caused by one of the following issues related to the setup of your Soapbox Donations for Salesforce package.

1) Ensure that you have followed all the steps in the Installation Instructions for Soapbox Donations for Salesforce guide, specifically any steps related to...

a) creating a default Lead record type

b) creating a default Campaign record type

c) creating a default Campaign Member record type

d) creating a default Opportunity record type