My Admob Test ads are working but real ads are getting error code 3 No ad config how to fix it

my AdListner getting error.. error saying ->

onAdFailedToLoad: {
      "Code": 3,
      "Message": "No ad config.",
      "Domain": "com.google.android.gms.ads",
      "Cause": "null",
      "Response Info": {
        "Response ID": "null",
        "Mediation Adapter Class Name": "",
        "Adapter Responses": []
      }
    }