Your Cloud9 environment will be used as the Fabric Client. In order for it to communicate with the blockchain network over the VPC endpoint, it needs to be in the same security group. To update its networking configuration, go to the EC2 Management Console and select Instances from the left sidebar, then check the box next to your cloud9 instance. Then select Actions, then scroll down to Security, then select Change security groups.
On the next page, begin to type HFClientAndEndpoint and then select it from the displayed options, then click AddSecurityGroup, then click Save.