Thank you for bringing this to our attention, @CLS.
To successfully import your customers into QuickBooks, please ensure their information is formatted correctly. You can use the linked sample file as a reference to guide you through the process.

Also, please let us know if you've encountered any error messages during the import process. This information will help us determine the best way to assist you.
If the problem persists, we suggest reaching out to Google's live support team for further assistance in investigating the issue and resolving why your client cannot be added to QuickBooks.
To learn more about importing your customers to QuickBooks, please refer to this article: Import customers or suppliers from email contacts to QuickBooks Online.
You can always return to this forum if you need further assistance.