Where can I report an extorsion email I received on my Imac today?

Extortion is a crime, it that's what it really is and not just some run of the mill phishing email you should report it to your local authorities.

Just as additional info: receiving such emails have nothing at all to do with your computer or, if it's a call, your phone. Both phone numbers and emails are regularly stolen and bought by crooks who try to make a living by trying to get to your money. And, since they are anonymous and keep changing their email addresses as well as phone numbers, it is very difficult to find them.

So, ignoring them is about the best thing to do.

Just know that many "extortion" emails are just scam emails that prey on the innocent not really knowing any better. An example would be an extortion email that displays a password you used at some point (from compromising a company and obtaining userid/passwords) and claims they read your personal email, or hacked your camera and caught you in compromising actions and to pay a "ransom" to prevent disclosures. They are scams and unfortunately, very little can be done.