I didn't know, or forgot, where to find error logs for ECS. If you're having trouble launching an app on ECS, you can find an error message as follows,
- AWS console
- ECS console
- Clusters pane
- Services table
- Select the service of interest
- Tasks pane
- Toggle STOPPED (from RUNNING)
- Select as Task
- Expand to see the error message
Hopefully, it's something simple to fix, like adding an inline policy in IAM.