@舜仁 胡 The most likely cause of this error is your resource is trying to scale to accommodate the rate of requests that are being triggered by your application. Please see this page on how you can handle the same.
You could also be using a free tier for your resource which has limited TPS for TTS and STT. This can be upgraded automatically by moving to standard tier and if you are already on standard tier and the error is still seen, please raise a azure support case to increase the quota for TPS for your resource.
I hope this helps!!
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