When getting permission denied errors, you can still try:
gcloud alpha resource-manager liens list

Which results in the error, but still gives the resource name, e.g.:
resourceName: projects/p815952589364-la3009308-1817-48d0-8ece-ca4d531dc7a3

Then use, which will delete the resource, even though it's complaining about permission:
gcloud alpha resource-manager liens delete p815952589364-la3009308-1817-48d0-8ece-ca4d531dc7a3

Now you can delete the google cloud project.