The Pro API requires you to authenticate all requests with an API Key. This needs to be added to your request in the form of a
apikey url parameter, or an
X-API-Key header. The API Reference has more info on authentication.
If you're still getting an error, you could have reached your rate-limit of 60 requests per minute. You can confirm this by checking the X-RateLimit-Limit, X-RateLimit-Remaining, X-RateLimit-Reset headers in the API response. More information on rate-limiting can be found in the API Reference.