Reports

It seems that there is not problem with this kind of css selector because it is used in official addons too (addons website_slides > slides_tour.js):

{
    trigger: 'iframe li.breadcrumb-item:nth-child(2)',
    content: Markup(_t("Click on your <b>Course</b> to go back to the table of content.")),
    position: 'top',
}

How about using div name=product_id to select the parent of your nth-child?

Is the problem maybe due to the fact that the modal content does not exist yet when performing your test ?

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