Let's say something like this is being returned when attempting to deploy a Task or Service in your Elastic Container Services (ECS) Cluster.

service flask-ec2-service was unable to place a task because no container instance met all of its requirements. 
The closest matching container-instance abcdefg123456789abcdefg123456789 doesn't have the agent connected.

I find this typically happens when attempting to deploy a service that will run on an EC2 instance. First and foremost, check to see if the EC2 Instance is up and running.

This can happen if you have two or more tasks/services using the same port. For example, perhaps you have a Flask and Nginx task definitions, both using port 80.

resource "aws_ecs_task_definition" "flask-ec2-task-definition" {
  family                   = "flask"
  network_mode             = "awsvpc"
  requires_compatibilities = ["EC2"]
  cpu                      = 1024
  memory                   = 2048

  container_definitions = jsonencode([
    {
      name = "flask-container"
      cpu       = 10
      memory    = 512
      image: "tiangolo/uwsgi-nginx-flask:python3.11",
      portMappings: [
        {
          containerPort: 80,
          hostPort: 80
        }
      ]
    }
  ])
}

resource "aws_ecs_task_definition" "nginx-ec2-task-definition" {
  family                   = "nginx"
  network_mode             = "awsvpc"
  requires_compatibilities = ["EC2"]
  cpu                      = 1024
  memory                   = 2048

  container_definitions = jsonencode([
    {
      name = "flask-container"
      cpu       = 10
      memory    = 512
      image: "nginx:latest",
      portMappings: [
        {
          containerPort: 80,
          hostPort: 80
        }
      ]
    }
  ])
}

And a Flask and Nginx service, both using port 80.

tbd

You can try giving each task/service a unique port to see if this resolves the issue.