Hi there @oleksandrHeras, 

Welcome to the Community! Glad you’re here!

Klaviyo doesn’t have any native documentation on how to install Back in Stock on collection pages as it requires a lot of custom coding. Since it requires custom coding, we cannot officially help troubleshoot how to get this set up on your site. However, a helpful workflow to set this up was narrated in this Community thread by a friend and might be a helpful place to start!

Additionally, while I can’t officially troubleshoot your tag issue, our help center documentation outlines that when tagging back in stock products on product pages, the tag calls out the specific product like ‘dog, cat’.

klaviyo.init({
    account: 'PUBLIC_API_KEY',
    platform: 'shopify',
    include_on_tags: 'dog, cat'
});

It’s possible you need to replace the ‘enable-back-in-stock’ tag with the collection names you want to target. However, to officially diagnose and set this up, I would recommend reaching out to a Klaviyo Partner to accomplish this.

Thanks for participating in the Community!

-Taylor