I’m following this tutorial. Now It’s work for me. Make sure you have banner ad test successful. For my observation, give some delay time for interstitial ad to load before call it to show. Not every times you call show, the ad is available. Sometimes the ad available is true but it’s don’t have the ad to show after you just show the ad a moment ago.
Not sure where to make it work but you have to try experiment yourself on internal test google play console. 
Anyway, thank you for your help.