Test Ads is not working in my application unless I changed application ID

One of my android applications (application ID: com.abc123.abc) is not receiving any test Ads unless I changed the application's ID which I don't want to do so because the application is released to the google play store. I am getting this error message "ERROR_CODE_NO_FILL" every time when I request the test Ads. When I change the application's ID to something else like "com.abc123.abcd", the testing Ads are working fine again. I'm using Android studio to conduct the request for the test ads. My source code is shown in the screenshots below. Any feedback would be appreciated.

1. .XML enter image description here

2. .java enter image description here