"errorMessage":[The INSERT statement conflicted with the FOREIGN KEY constraint

I have an issue with the this:

Post //WebConsoleHostName/webconsole/api/ClientGroup/clientGroupId 

{
   "clientGroupOperationType": "Update",
   "clientGroupDetail": {
   "clientGroup":{
       "clientGroupName":"smarties"
     },
      "associatedClientsOperationType": 2,
      "associatedClients": a
         {
            "clientName": "smarties-test01"
         }
      ]
   }
}

And I recieve the error:
{
"errorMessage":"INFO: eThe INSERT statement conflicted with the FOREIGN KEY constraint \"APP_ClientGroupAssoc_clientId_FK\". The conflict occurred in database \"CommServ\", table \"dbo.APP_Client\", column 'id'.] RecNum:1, Object:APP_ClientGroupAssoc, State:23000, NativeErr:547, RowNum:1, Severity:16, LineNum:1, Spid:-1] INFO: ,The statement has been terminated.] FRecNum:2, LineNum:1, Spid:-1] ","errorCode":1016}

Any ide what is the problem here!