Unable to delete network interface

That happens when the interface (ENI, elastic network interface) is owned by an AWS service. For example, if the ENI belongs to a NAT gateway, load balancer, VPC endpoint, or RDS database instance, you can't and shouldn't try to delete the network interface directly. Instead, you should delete the NAT gateway or other resource, and the service will delete the interface associated with it automatically.

You can identify the resource typically from the "Description" field of the ENI. For example, for a NAT gateway, it'll show Interface for NAT Gateway nat-01234567890abcdef that identifies the specific NAT gateway.