One (or more) of the following problems is applicable:
Problem scenario #1
You are trying to delete an network interface in AWS. But you get an error "You are not allowed to manage 'ela-attach' attachments." What should you do?
Problem scenario #2
You are trying to delete a subnet from AWS' VPC. But you get this error:
Note that the following subnets cannot be deleted: The following subnets contain one or more network interfaces, and cannot be deleted until those network interfaces have been deleted. Click here to view your network interfaces. subnet-1234abcd | foobar
What should you do?
Possible Solution #1
Go to Lambda functions. In the "Configuration" tab of the function, search for the "VPC" section. Detach the relevant VPC. (This can easily prevent the deletion of a Network Interface or Subnet.)
Possible Solution #2
Remove unused VPC links from the API gateway.
Possible Solution #3
Remove unused VPC Endpoint services.
Possible Solution #4
Remove unused NAT gateways.
Possible Solution #5
Remove unused ECS/EKS clusters.
Possible Solution #6
Remove unused load balancers.
Possible Solution #7
Remove unused Elastic IP addresses (go to EC2 -> Network & Security).
Possible Solution #8
Remove unused Route Tables.
Possible Solution #9
Remove unnecessary Internet Gateways
Possible Solution #10
Remove unnecessary Endpoints.
Possible Solution #11
Remove unnecessary Peering Connections
Citation: Possible solutions #2 through 6 came from this StackOverflow.com posting.