https://developer.salesforce.com/forums/?id=9060G000000I7sYQAS ,and “D”.( When importing data into the NPSP Data Import Object, if the error message indicates that the Record ID field is not mapped, the system admin should ensure that the appropriate mapping is set up and use the correct operation for the data loader.

Select the Appropriate Record ID (A):

In the Salesforce Data Import Wizard, ensure that the Record ID field is correctly mapped. This field is required for updating existing records or ensuring data integrity.

Select "Upsert" Operation (C):

When using the Salesforce Data Loader, choose the "Upsert" operation. This allows the system to insert new records or update existing ones based on the Record ID or an external ID, ensuring that duplicates are avoided and data is accurately imported.

Steps to Resolve:

Check Field Mapping:

Verify that the Record ID field is mapped correctly in the import wizard or data loader.

Choose Upsert Operation:

If using the Data Loader, select the "Upsert" option to handle both insertions and updates.