Can I send money via Apple Cash to someone in Greece?

See the following from Send and receive money with Apple Cash - Apple Support

To send and receive money with Apple Cash, you must be at least 18 years old and a resident of the United States. If you're under 18 years old in the United States, your family organizer can set up Apple Cash for you as part of Apple Cash Family. Then, you can send and receive money with Apple Cash. Apple Cash services are provided by Green Dot Bank, Member FDIC. Learn more about the Terms and Conditions.

Probably not.

BOTH the sender and receiver must have AppleCash Cards … and they are “U.S. Resident” only.