Error 'Required Fields Are Missing' When Importing Records with the Salesforce Data Import Wizard

This error occurs when a master-detail field is mapped during import but the CSV file contains no data in that column. Since every master-detail field is required on the detail (child) record, the attempt to create records fails.
To resolve this error, populate the CSV file with the ID of the associated parent record in the master-detail field column, then attempt the upload again.
Example: Suppose you are importing records into a custom object called "Order Line__c" that has a master-detail relationship to a parent object called "Order__c". Your CSV file must include a column containing the Order__c record ID for each row. If this column is empty or missing, Salesforce cannot create the child Order Line__c records and returns the REQUIRED_FIELD_MISSING error.
Steps to resolve:

1. Open your import CSV file.
2. Locate the column mapped to the master-detail relationship field (e.g., Order__c ID).
3. Populate each row with the 15-character or 18-character ID of the corresponding parent record.
4. Save the CSV file and re-run the import using the Data Import Wizard.