You have been targeted by the FBI virus. You don't have a Federal case against you, and you have done nothing wrong. This specific virus is attached to many porn sites. It then gives you a message that looks reasonably official and is designed to scare you into thinking you are going to be arrested so that you will run out and get some kind of cash card to prevent an arrest.
DO NOT SEND THESE FOLKS ANY MONEY. They are just criminals and they have nothing to do with the law. Porn is legal as long as the on-screen porn stars are at least 18.
When the government really wants you, they don't give you advance notice so that you can run out and destroy your phone or remove its SIM card. They do not block your cell phone or ask you for a PayPal payment to avoid prosecution. What they will do is knock on your door with a warrant and impound all of your electronic devices that can access the internet.
So, again, do not pay the fee. Nobody is going to arrest you. You have broken no law. What you DO need to worry about is that this scam is also a virus. And you need to get it off of your phone. So what you may want to do is to post another question here to our mobile phone category and ask them how you remove the "FBI virus."
There are instructions how to do it on line. Simply google "FBI virus phone" to see available solutions. Here is one that explains the bug and tells you how to remove it. If you are not particularly tech-savvy and want to be sure it's gone or you don't want to trust a strange website, you can also call your phone carrier and see what they recommend, as I am sure they are aware of this particular problem. Alternatively, you can post a new question to one of our verified mobile phone techs here on JustAnswer.
But you can breathe easy. You're not going to get arrested and you don't have to find a My Cash card and send anybody money. Report the fraud on the FBI's online Internet Crime Complaint Center at IC3.gov and to the Federal Trade Commission at FTC.gov.