AWS ECS Error when running task: No Container Instances were found in your cluster

I figured this out after a few more hours of investigating. Amazon, if you are listening, you should state this somewhere in your management console when creating a cluster or adding instances to the cluster:

"Before you can add ECS instances to a cluster you must first go to the EC2 Management Console and create ecs-optimized instances with an IAM role that has the AmazonEC2ContainerServiceforEC2Role policy attached"

Here is the rigmarole:

1. Go to your EC2 Dashboard, and click the Launch Instance button.

2. Under Community AMIs, Search for ecs-optimized, and select the one that best fits your project needs. Any will work. Click next.

3. When you get to Configure Instance Details, click on the create new IAM role link and create a new role called ecsInstanceRole.

4. Attach the AmazonEC2ContainerServiceforEC2Role policy to that role.

5. Then, finish configuring your ECS Instance.
NOTE: If you are creating a web server you will want to create a securityGroup to allow access to port 80.

After a few minutes, when the instance is initialized and running you can refresh the ECS Instances tab you are trying to add instances too.

Currently, the Amazon AWS web interface can automatically create instances with the correct AMI and the correct name so it'll register to the correct cluster.

Even though all instances were created by Amazon with the correct settings, my instances wouldn't register. On the Amazon AWS forums I found a clue. It turns out that your clusters need internet access and if your private VPC does not have an internet gateway, the clusters won't be able to connect.

The fix

In the VPC dashboard you should create a new Internet Gateway and connect it to the VPC used by the cluster. Once attached you must update (or create) the route table for the VPC and add as last line

0.0.0.0/0 igw-24b16740  

Where igw-24b16740 is the name of your freshly created internet gateway.

I ran into this issue when using Fargate. I fixed it when I explicitly defined launchType="FARGATE" when calling run_task.