The Subnet ocid1.subnet.oc1.xxx references the VNIC ocid1.vnic.oc1.yyy. You must remove the reference to proceed with this operation.
Typically, you have an instance in there … or a volume mount. Not here for us. The VNIC belongs to a primary ip … without further info. No instances.
What was it ?
We had a cloud-shell with private network access in that subnet – and somehow it keeps the VNIC. Only solution was – start the cloudshell again privately – and change the network to public. Then it took some time … but after 20 minutes or so the VNIC was magically gone.
Oracle, Exadata, Multitenant and related 