Is it possible that you are using the Nonprofit Success Pack Individual, or "Bucket", Account model but no "Individual" Account record has been created? This could be the source of this error.
To fix go to Accounts > New > Create a new account with the title of "Individual" > save
That should do the trick!
For more information of Nonprofit Success Pack Account models, check out NPSP: What is an Account Model? from Salesforce.org.