"No ad config" when trying to load RewardedAd

I'm following this guide here for showing rewarded ads in my application.

At the end of my OnCreate method, I call the following:

private fun loadAdvertisement() {

    // Test Ad
    rewardedAd = RewardedAd(this,
        "ca-app-pub-3940256099942544/5224354917")

    val adLoadCallback: RewardedAdLoadCallback = object : RewardedAdLoadCallback() {
        override
        fun onRewardedAdLoaded() {
            Log.d("Ads", "Loaded Ad")
        }
        override
        fun onRewardedAdFailedToLoad(adError: LoadAdError?) {
            Log.d("Ads", "Failed to load Ad")
        }
    }

    // Get Personalized Ad Consent
    val consentInformation = ConsentInformation.getInstance(applicationContext)

    // If Consent is required or unknown, use non personalised
    val request = AdRequest.Builder()

    val extras = Bundle()
    if (consentInformation.consentStatus != ConsentStatus.PERSONALIZED) {
        extras!!.putString("npa", "1")
        request.addNetworkExtrasBundle(AdMobAdapter::class.java, extras)
    }

    rewardedAd!!.loadAd(request.build(), adLoadCallback)
}

The Ad load fails with the following error:

{
  "Response ID": "null",
  "Mediation Adapter Class Name": "",
  "Adapter Responses": []
}

zzacn: com.google.android.gms.ads
zzacm: No ad config.
Code: 3

Can anyone see where I'm going wrong here?

Edit

Changing my projects namespace appears to allow test ads, anyone know why that might happen?