Once you create a Sandbox by copying over a production environment and trying to make HTTP requests, it gets blocked to prevent the use of production services.
If you want to enable the requests going through, go to the Extension Management page and select the extension initiating the HTTP request. Go to the settings of the extension. You will probably notice that “Allow HttpClient Requests” is disabled, and you need to enable it. Please refer to the below two screenshots.
Once you enable the “Allow HttpClient Request” the sandbox will allow outgoing HTTP requests and will not block them.
Thank you and Regards,
Tharanga Chandrasekara