Send Emails - "Insert Placeholder" Button Not Working

Hello! I'm trying to use Kutools to send e-mails to 146 people, each with a separate attachment customized to each individual. I followed the instructions found here: https://www.extendoffice.com/product/kutools-for-excel/excel-create-mailing-list-and-send-email.html.

I've created the mailing list in Excel as instructed.  I've followed the instructions for actually sending the e-mails; but, when I get to step #2, the "Insert Placeholder" button does not work.  No matter what field I select (e.g., "C: First Name," "D: Last Name," etc.), nothing happens.  I can type directly in the e-mail body, but I can't get any placeholder fields to insert.  I've followed all of the instructions exactly and can't see why it wouldn't work.

I've attached a screenshot of the Send Emails dialog box so you can see I have everything set up correctly.

Any ideas?