You get this error when the file you are attempting to upload is not in a UTF-8 format. UTF-8 is the dominant character encoding for email and the web. Character encoding tells your web browser how to display special characters like ' (apostrophe) or - (hyphen).

This error occurs because the software you are using (Microsoft Excel, for example) saves the file in a different type of encoding, such as ISO-8859, instead of UTF-8. This will cause your web browser to show special characters like ' and - as ?.
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There are a few solutions you can use to change your file to UTF-8 encoding. If your file already has ? instead of the correct characters, you will need to fix these manually before saving your file.
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If you have a Gmail account or have used Google Drive previously, you can:

1. Open up Google Drive.
2. Upload your your CSV file.

3. Open your file as a Google Sheet.
4. Navigate to File>Download As>Comma-Separated Values to save the file to your computer.
5. The file should now be in UTF-8 encoding, and it will successfully upload.
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If you’re using the Windows operating system and you have Notepad, you should be able to perform these steps to open your Excel file in Notepad and change the encoding:

1. Open the CSV file using Microsoft Notepad.
2. Navigate to File > Save As option.
3. From the Encoding menu, choose UTF-8 and save the file.
4. The file should now be in UTF-8 encoding, and it will successfully upload.

1. Open the CSV file with Microsoft Excel sheet.
2. Navigate to the File menu option and click Save As.
3. Click Browse to select a location to save the file.
4. Select the Save as type as CSV (Comma delimited) (*.csv) option.
5. Click Tools drop-down button and choose Web Options. A new window for web options appears.

6. On the Encoding tab, select the option Unicode (UTF-8) from Save this document as: list.

7. Click Ok, and save the file.
8. The file should now be in UTF-8 encoding, and it will successfully upload.