Where is the area code 599 for the US?

There is no area code +1 599 in North America (USA, Canada, etc.); area codes cannot have '9' as the middle digit.

International telephone country code +599 is used for certain Dutch islands in the Caribbean, including Curacao, Bonaire, Sint Eustasius, and Saba. The Dutch part of the island of Sint Maarten formerly used +599, but now uses North American area code +1 721. Aruba uses country code +297.

(The plus sign means "insert your international access prefix here." From a GSM mobile phone, you can enter the number in full international format, starting with the plus sign. The most common prefix is 00, but North America (USA, Canada, etc.) uses 011, Japan uses 010, Australia uses 0011, and many other countries use different prefixes.)