ERROR WHILE USING POST METHOD IN INTEGRATION

HttpResponse
    statusLine: "HTTP/1.1 200 OK"
    statusCode: 200
    headers: Dictionary
        Date: "Thu, 07 Sep 2023 12:24:23 GMT"
        Content-Type: "application/json"
        Content-Length: "317"
        Connection: "keep-alive"
        Server: "Apache"
        Access-Control-Allow-Origin: "*"
        Access-Control-Allow-Headers: "origin, x-requested-with, accept, content-type, Authorization"
        Access-Control-Max-Age: "3628800"
        Access-Control-Allow-Methods: "GET, PUT, POST, DELETE, OPTIONS"
    contentType: "application/json"
    body: "{
  "Apttus__APTS_Agreement__c" : {
    "success" : "false",
    "errors" : {
      "message" : "Upsert Agreement Failure:\n\nError Status Code: INVALID_FIELD;\nError Message: OBJ: Apttus__APTS_Agreement__c - Foreign key external ID: CO-UK-977-6935 not found for field Name in entity Protocol__c\n"
    }
  }
}"

Hello All, 

We are getting this above error while sending data to third party system using POST MENTHOD. What might be the issue?