Can I resend a Back in Stock notification? (Essent Preorder Back in Stock app)

No. Each sign-up gets one email, sent automatically when the variant goes back in stock. There is no button to send it again.

If customers missed it:

Ask them to sign up again with Notify me. They get an email at the next restock.
Or export the list (Back in stock → Export notifications) and email them from your own email tool.

To check only the email design, open the campaign's Email tab and click Send test email.