I am confused about your problem - you used a Fiverr seller for “tutoring”…in English / the US that means someone going over math, science, etc. and explaining difficult concepts to you. Then you say that the seller is going to report you for plagiarism, which again, in English / the US that implies a written piece of work that has been stolen and passed off as someone else’s.
Did you hire someone to write a school essay for you, and now he’s threatening to go to your school/college and tell them you hired him to write for you? If so, you should understand that both of you did something wrong here - you are not allowed to buy or sell academic work on this site, whether you call it “tutoring” or something else. The spirit of the rules are that if you’re hiring someone to do your schoolwork for you, you both either get a warning or removed from the site.
Secondly, the TOS prevents contact off-site. They want all discussion and contacts to go through Fiverr, otherwise they have no way of monitoring deliveries and protecting buyers and sellers. And they’re rather protective of their 20%, naturally.
Finally, you say he “never delivered” - he never gave you any document at all? Never provided you with any work? If so, this is a non-issue and I’m not sure what he thinks he’s blackmailing about, or what you’re worried about. It’s like threatening to have someone arrested for shoplifting from a store they’ve never been to.