Attach Volume EFS in ECS
You need to open port 2049 inbound on the security group on the network interface and task definition. It was not automatically set up even though If you set it to create the security group for you.
It's been a while now but I've had the same issue and it was a bit confusing to understand how to proceed. When you create your EFS Volume, you choose a VPC and one Security Group to each Subnet.
You need to go to edit this Security Group to add an Inbound rule of type NFS to allow access (tcp port 2049) to the Security Group Identifier of your ECS cluster service that you want to allow access to. For that, just select Custom in the source field and type service's Security Group identifier on the text box.
For more information this article describes the whole process very well.
If you enabled IAM Authorization while associating the Task Definition to the volume, you also need to update its Task Execution Role. You need to attach the policies required to access EFS to it.