When this happens
The request exceeded a limit. Each API key may send 60 requests per minute and run one request at a time. In sandbox, the same limits also apply to each Client across all of its keys. Every request counts, includingGET.
What to do
Wait for the number of seconds inRetry-After, then retry the unchanged request. Concurrency rejections return Retry-After: 1. Per-minute rejections return Retry-After: 60. Send requests for one key one at a time. See Rate limits.