Steven Zahm@ Carl
The Unexpected token < in JSON seems to indicate that something is causing a PHP error and PHP is displaying the error in the import response which breaks the process.
While in the Console tab in Chrome, after that error occurs, you can click the Network tab. near the end of the list you’ll very likely find a red line item. Clicking that line and then clicking the Preview or Responce tab will display what the server actually sent as a response. Knowing that will help to know what the source of the error is.
If this is the cause of the import failure, disabling the display of PHP notices may help with this. This is generally considered to be a best practice on live sites. Here’s a link to a tutorial which will help with that.
How do I hide PHP Warnings and Notices?
You can also try doing the import with all other plugins temporarily deactivated to eliminate them as the source.
Also, make sure you closely follow the CSV file requirements and tips. Using the header example Excel sheet is not enough.
https://connections-pro.com/documentation/csv-import/#CSV_File_Setup_Requirements
Lastly, I can take a closer look if you want. Please create a temp admin account using this plugin:
https://wordpress.org/plugins/temporary-login-without-password/
Reply back, as a private reply, with the login link. Also, please attach a copy of your trest CSV file. And, attach a screenshot of the mapping you are using to apply to the import.