When you say it is unable to detect set > content value what do you mean? Looking at the flow you are using the node name of HTTP Request1 but the HTTP Request node is called HTTP Request so it could just be that… Another option as the Set is after the request node would be to just use {{$json["choices"][0]["text"]}}
The HTTP Request node is not called HTTP Request1 in your workflow so you would need to change the name or instead try using {{$json["choices"][0]["text"]}}