Thank you for reaching out to Constant Contact API Developer Support. I'm posting this here as well in case anyone comes across this post and does not see your other one.
This error means that you have reached your daily call limit. Our standard API rate limit is 10,000 calls per day, and 4 calls per second. Once your key has hit its limit, your calls will see this error until the next day (or the next second if going over your queries-per-second limit)