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

I'm trying to complete the challenge Build an Event Registration App

in Trailhead. I complete all the steps to create the Event Registration Object and the needed custom fields (all values/types are correct and validated) 

Then I tried importing the data needed so I downloaded the ImportFile2023.csv and proceed with the Data Import Wizard.  

Following the steps, I manage to map the fields and leaving the First Name and Last Name unmapped as instructed. But upon importing, I keep on receiving the error of "

" 

I doubled checked: 

1. The file has the Attendee column with values. 

2. Attendee custom field is existing 

3. Attendee has been mapped correctly. 

I retried the process but still getting the same error. Anyone experienced the same? 

Thanks!