Conflict - The Subnet ocid1.subnet.oc1... references the VNIC ocid1.vnic.... You must remove the reference to proceed with this operation.

 Recently , i was cleaning Oracle OCI Account. I delete / terminated most of things 
But subnet was giving error 

Conflict - The Subnet ocid1.subnet.oc1.iad..xxxxxxxxxxxxx   references the VNIC ocid1.vnic.oc1.iad.xxxxxxxxxxxxxxxxxxxxxxxxxxx . You must remove the reference to proceed with this operation.

After searching came across following documenation 

  See the following for official Oracle directions covering the process to delete an Orphaned Mount Target – https://docs.cloud.oracle.com/iaas/Content/File/Troubleshooting/orphanedmounttarget.htm

But it not help , as i was not using File system. But it gave clue that i can query VNIC on OCI CLI  and see what attached to it 

when query VNIC , we saw one MYSQL instance not terminated yet 

root# oci network vnic get --vnic-id ocid1.vnic.oc1.iad.abuwcljsyovdlywqyd7tndgwfdfj2pbwo76gfdh6iocr4g552xmgqucy3hnq  
{
  "data": {
    "availability-domain": "QNKP:US-ASHBURN-AD-1",
    "compartment-id": "ocid1.tenancy.oc1..aaaaaaaajjkprkd7xdzwso4f6lki6mj7nz5xqnoo7rwurtvtxb56spycbtsq",
    "defined-tags": {
      "Oracle-Tags": {
        "CreatedBy": "mysql",
        "CreatedOn": "2021-06-10T06:34:44.152Z"
      }
    },
    "display-name": "vnic20210610063448",
    "freeform-tags": {},
    "hostname-label": null,
    "id": "ocid1.vnic.oc1.iad.abuwcljsyovdlywqyd7tndgwfdfj2pbwo76gfdh6iocr4g552xmgqucy3hnq",
    "is-primary": false,
    "lifecycle-state": "AVAILABLE",
    "mac-address": "02:00:17:0B:D9:98",
    "nsg-ids": [],
    "private-ip": "10.0.1.196",
    "public-ip": null,
    "skip-source-dest-check": false,
    "subnet-id": "ocid1.subnet.oc1.iad.aaaaaaaafekmxhstcrmqttpfxeakojqtqvlor6ik6qmtqskxawecrhtiovgq",
    "time-created": "2021-06-10T06:34:48.386000+00:00"
  },
  "etag": "3e6a18c3"
}