Plagiarism is a heavy offence and is 100% illegal, especially on Wattpad with their Canadian laws. As a past ambassador there, my best advice is to first try and talk to the person who plagiarized your story and ask them to take it down. If they do not listen, you must contact the Wattpad staff asap. They will solve the issue, but you must include as much proof of the plagiarism if possible.
Saying all that, this only applies if someone plagiarized your work word by word (or only changed names, certain wordings, etc...) Proving the plagiarizing of ideas is much harder however. So it depends what exactly did this person plagiarize. Not saying that this is the case, but if they only have the same concept for a story but wrote it themselves, it does not count as plagiarism since they could claim it was inspiration. (or if the idea is widely used in fiction, they could just say that it's a coincidence)