Q: Can I withdraw my paper after coming to know that the journal doesn't have an ISSN?

This seems to be a predatory (bogus) journal, and they will now adopt their usual tactics of trying to force money out of the researcher. You may read more about such journals and their tactics here: What should I do if I mistakenly submitted to a predatory journal and they are now using coercive tactics?

The thing is, it’s not entirely clear from your details whether they have actually published the article on their site or have simply shown you a PDF version. In case it’s only a PDF version, you have a chance of withdrawing the article. If it has already been published, you can’t technically withdraw it, but would have had to ask them to remove it from their site.

To answer your question, you don’t have to pay, but they will probably then threaten to publish your article (if they haven’t) or threaten action against you (by reaching out to your supervisor/institution). You should stay firm, involving your supervisor as needed. Don’t worry, new/junior researchers becoming victims of predatory journals is fairly common, and your supervisor will probably understand. In fact, they would probably be able to help you out of this situation.

However, if the article has already been published or ends up being published, it would be very difficult to have it removed, unless you are willing to pay a hefty fee for that. Our suggestion would be to try your best to not let them publish the article, but if they do end up doing so and you can’t have it removed, let this article go as a learning experience. And to avoid such situations in the future, you may keep this checklist handy: 10 Point checklist to identify predatory publishers

Hope that helps. All the best for a smooth resolution!