What I did when this happened was, while the resource creation was in progress, I went to ECS in the AWS Console, opened up the cluster with the problem and went to the "Tasks" tab. In there, you can click on a failed task and a message should pop up at the top saying why the task failed to initialize.

https://repost.aws/knowledge-center/ecs-service-stuck-update-status also says you can look at the "Events" tab for the cluster's service. The events view didn't show me any errors, but supposedly sometimes it does.