The error you received has the instruction you need to follow: setting the API key environment variable in the OS, or hard-coding it as `client(api_key=“sk-mykey”)
If you’re feeling adventurous after getting your first mp3 and seeing how path works, you can follow this link to a version with a ttsFile class that maintains settings and would automatically produce timestamped files using a prefix. While it works as is, with a prompt to edit at the bottom, the entire if __name__ == "__main__": is simply example of how to use the class.
It saves to the present code directory with a prefix that is just part of the file name, but a full path and filename prefix could be used.