Not all beta applications are accepted. Also, you don't need a beta license to do beta testing, just enable the beta or nightly update channel. I would not be surprised if the beta testing program goes away at some point as it's not needed anymore because of the addition of the beta and nightly channels, which anyone can use and you don't need a special beta tester license to use. But, of course, you'd still need a valid AdGuard license to use it regardless.
If purchasing a license directly is too expensive, I suggest looking into the killer deal for AdGuard licenses on StackSocial. You can get licenses on the cheap there, and it's a valid offer/discount.