That is a message reserved for a problem with your payments and billing. There is no funding to pay for your API call by the organization specified or associated with the API key.
Currently it’s usage tier 1. But I looked through that tier’s limits, and don’t think I’ve hit them… But I guess I could be wrong. Do you happen to know a way to find out WHICH limit I’m hitting?
I’d go to the account billing overview, and check your available credits. Also the limits page where you can set your own hard limit to cut off usage per month.