In AWS sometimes you could see the error message when try to detach ENI “Error deleting network interface” . It’s normal , we just need to understand the concept . ENI’s are not only created by manually . When you create some resources , AWS will automatically create the backend must required resource.
Example when you create EFS it will create ENI’s in multiple AZ automatically . So when you try to delete ENI directly it wont allow you to delete .So first you have to delete EFS targets ,then you can detach or delete the actual resource.
This scenario not only for EFS , same for NAT gateway , FSX or some more resources also. You have to find out backend dependency tree for any kind of resources .
Always you can check Elastic network interfaces section in “EC2” or “VPC” section for existing ENI details.