The issue is that Cloud9 can’t directly connect to the private address of a server in a private subnet unless both are in the same VPC with proper internal routing. Usually, it’s enough to make sure Cloud9 and the EC2 instance are in the same VPC and that the server’s Security Group allows access from Cloud9. If Cloud9 is in a different VPC, you’ll need to set up VPC peering or a similar connection between them.