ECS will not launch instance, "unable to place a task because the resources could not be found."
my guess is that your cpu units is too high. https://docs.aws.amazon.com/AmazonECS/latest/developerguide/task_definition_parameters.html#container_definition_environment
it's a harder metric to guess if you haven't really measured it much on your app.
anyway, i'm hitting a similar issue, so i'm right there in the same boat, but i'd try erasing the cpu since it is optional and see if that resolves it.