Most of you run Facebook ads which means that your customers opens your store inside Facebook native app. Web Pushes does not work there. Your customers can't subscribe inside Facebook app. 

What to do?
You have to make sure your ads will be opened in a default users browser.

2. Fill in all required information up to section Ad > Links

3. Find App link destination dropdown. If you leave it as it is all your ads will be opened inside Facebook native app.
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4. Click on App link destination dropdown and choose Open deep link in a mobile web browser. Users will be directed to their default web browsers to view a deep link.

Please note that Web Pushes do work on:

Browsers:

Web pushes do not work on iOS devices such as iPhones and iPads. Apple does not support pushes at the moment. However their should do that in near future.