Ad is not displayed for my ad unit Id in my test device

I have created my ad unit ID a day before in AdMob and also added test device as AddTestDevice("da****.****") in the AdRequest.Builder() function. But I am not getting ad in banner. What can be possible reason for it? I am still in development phase and do I need to setup my payment account in AdMob to get ad in testing phase? Meanwhile, using sample ad unit ID from the tutorial of AdMob displays test ad in the simulator.