Q: What is the legal action a journal can take if I don't pay the fee?
My article has been published in a journal, and now, they are sending me mails to pay the fee. If not, [they have said that] they will take legal action. If I don't wish to pay the fee, what will happen? What is the legal action the journal can take?
From the information you have provided, the sense we get is that this is a predatory or bogus journal. It seems you didn’t initially know this (when submitting), but figured it out later, by when the article was already published. This is the usual case with a predatory journal. They publish articles with little, token, or no peer review, and therefore publish them fast, to capitalize on the academic pressure to publish.
Anyhow, you seem to have wised up and also seem to be quite matter-of-fact about it. You probably realize that the article is ‘gone’ (that you can’t now publish it in a legitimate journal). Also, you don’t seem too disturbed by the ‘threats’ of the journal. Actually, these threats are more like coercive tactics.
To answer your question, they can’t really take any legal action, because they are the ones on the wrong or gray side of the law. At the most, they’ll threaten to inform your supervisor or institute, but that’s something you’ll need to do on your own as a way of acknowledging your error of judgment. Don’t worry, your supervisor/institute know(s) that many a researcher falls prey to such journals and are not likely to censure you. In fact, they will probably support and help you, if it comes to that.
Another ploy the journal can use is to ask you to pay a fee to withdraw your manuscript. From our experience though, these threats or tactics usually subside after a while. This of course is assuming this is a bogus journal. To double-check which, you could refer to this checklist: 10 Point checklist to identify predatory publishers
If on the other hand, this is a genuine journal, you would need to do the ethical thing yourself and pay the fee. For our response to a similar deliberation by another researcher previously, you may refer to this question: Do I still need to pay the APC in case I don't receive a payment reminder?
Hope that helps. All the best either way – in dealing with the predatory journal or in doing the right thing and making the payment!
For more on dealing with predatory journals, you may refer to these previous queries by other researchers: