The powershell error looked more like a network error. So I had suggested running the below command :
[Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
$headers = @{"Ocp-Apim-Subscription-Key" = "<Your Subscription key>";"Content-Type"= "application/json";"charset"="UTF-8"}
$response = Invoke-WebRequest -Uri "https://api.cognitive.microsofttranslator.com/translate?api-version=3.0&from=en&to=zh-Hans" -Headers $headers -Body "[{'Text':'Hello, what is your name?'}]" -Method Post
The code [Net.ServicePointManager]::SecurityProtocol = [Net.SecurityProtocolType]::Tls12
enforces to make use of the TLS 1.2. As mentioned in this article, Cognitive Services endpoints exposed over HTTP enforce TLS 1.2.
Translated code ServicePointManager.SecurityProtocol = SecurityProtocolType.Tls12;
helped you to resolve your Issue.
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