The issue is caused by the customization code you added to the child theme. Because when I go to the child theme function.php file and then I comment the code you added, your issue was resolved now the popup appears just fine right after the add to cart product.
I have made a complete process video for you kindly watch the complete video: https://streamable.com/6yj5cr
Please check your customization code and correct it in order to solve your issue.
Regards 8Themes Team.
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