These are the things that I can think of to check/try out:
1. Try to make a new project in C2 with a single button to show a banner.
And create a new banner (auto size is what I chose) ID in the admob portal.
2. Check if you are not copying an empty space at the beginning or end of the admob ad id.
3. Try downloading the latest C2 release.
4. Your app must be signed by ticking that box in XDK.
5. place the banner at the top center (because I did not test out other positions )
6. Check this topic (also first 2 or 3 pages with screenshots)
1. I have done exactly that with but recieved the same results
2. I have checked that over and over again, the id is correct
3. I just downloaded construct and Intel XDK onto another computer a couple hours ago, same results
4. App is signed
5. I believe I have tried this but I will try again (I have tried other positions besides bottom center with the same results
6. I've checked that topic over and over and even replied there to see if I could get help but I believe they have a different issue :/
There has to be SOMETHING that I'm missing right? I swear I have done everything I can find on the Internet to solve this.