Connecting S3 bucket through WinSCP

It seems WinSCP is unable to resolve the S3 URL. As you have mentioned since this is an on-prem desktop the most probable cause may be a block in the instance firewall or outbound traffic is blocked at a network level.

You can add an exception in the Windows firewall as described here -> https://winscp.net/eng/docs/faq_connection_refused. I would also recommend trying out a different client if possible to check if you can establish a connection.