This can be if you are missing ServiceProviderAPI scope. And you cant find this scope at tenant level.
On Managed clusters Network Zones are cluster wide hence they can not be deleted at tenant/environment level. They have to be deleted at cluster level using Cluster API.
To get an API-Token with the ServiceProviderAPI scope, go into the CMC of the Cluster under Settings then to API Tokens. Click on Generate Token under Cluster Token, give it a name and enable ServiceProviderAPI for the access scope.
The new generated token should now be able to authenticate for the deletion REST API endpoint for Network Zones. Which you can use to delete the network zone.