When you attempt to import records into a custom object, you may encounter the error message:
"REQUIRED_FIELD_MISSING:Required fields are missing: [Fieldname__c]:FieldName__c --"
This error occurs when the field in question is a master-detail field, and the field has been mapped during import but contains no data in the CSV file. Since every master-detail field is required on the detail, or child record, the attempt to create records fail with the error above.
To resolve this error, populate the CSV file with the ID of the associated parent record and attempt the upload again.
See also
General Importing Questions
Prepare a CSV file for an import or update in Salesforce
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