Cuba requires non-U.S. medical insurance. This is not a travel insurance policy or protection against trip cancellation. Cuban medical insurance is to cover you in case you become ill while in Cuba and require medical assistance.
There are agents who will sell you a temporary policy upon arrival at the Havana airport for a reasonable rate (roughly $3 USD per day). When traveling direct from the U.S. the price of your flight includes a Cuban health insurance policy (valued around $46).