If you don't see the "Custom Field Mapping" window as shown in the screenshot when uploading customer data, please check if your file has the following issues:

 

1. The first row of the file must not be empty and should contain column names.
2. The imported file should be placed in the first worksheet.
3. Confirm that there are no frozen panes in the file.
4. Avoid merged cells.

By addressing these issues, you should be able to see the "Custom Field Mapping" window and proceed with mapping the fields appropriately.