The network interface can't be deleted

Hello.

Isn't that ENI used by another AWS resource?
For example, if ALB or NAT Gateway is being used, you need to delete ALB etc.
Therefore, please check if there are any remaining AWS services running on the VPC.
You can search for active services in AWS by following the steps "Confirm you have terminated all active resources" in the document below.
https://repost.aws/knowledge-center/check-for-active-resources

Also, please check the following documents as they may be helpful.
https://repost.aws/knowledge-center/troubleshoot-dependency-error-delete-vpc

Hi,

This script provided by AWS allows you to find resource still linked to an ENI: https://github.com/awslabs/aws-support-tools/tree/master/Lambda/FindEniMappings

You will be able then to find the attached resource and destroy it before destroying the ENI.

Best,

Didier