Help with Import Data for Event Registration module

In the "Build an Event Registration App", the data import fails with an error, 

"REQUIRED_FIELD_MISSING:Required fields are missing: [Attendee__c]:Attendee__c --"

EDIT: Solution: create a new playground and build the project in the new playground.

The file has the Attendee column, the Event Registration object has the Attendee field. Column is mapped. The other data columns import. The file looks ok with comma delimiters. I'm on a Mac, the line endings may be different, but again, the other columns import. What is causing this error?

edit: This is an import process that's having a problem, not a program. This is from a Salesforce Project so you can load it up and try it yourself. The Event_Registration.Attendee__c field is a related field to the Contacts object, not a text field or other scalar. Whatever the built-in constraints are, I do not have the option to remove any "Required" flags. 

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