PeerPlay
PeerPlay has a Report an Issue button on every app a tester joins. It offers three choices: Broken Download Link, Google Group Error, and Not Available in My Country. The third one exists because a tester can do everything right, accept the invite, open the link, tap install, and still be told the app is not available to them.
For Google Play's 12 testers, 14 days requirement, that tester doesn't count, no matter how willing they are. So before you count someone toward your 12, check four things: their account, their country, their device and their account type.
1. A Google account on your list
Every closed tester needs a Google account, and that account has to be on your tester list, either added by email in Play Console or as a member of a Google Group you attached to the track. Then they have to accept through your opt-in link.
The common trap is a phone with more than one Google account. The tester opts in with one account in the browser and the Play Store app is signed in with another. Ask testers to opt in with the same account they use in the Play Store on that phone.
If you use a Google Group, check that its join settings actually let testers in. A group that only accepts invited members, or one where join requests wait for an approval nobody gives, stops testers before they ever reach your opt-in link. That is the problem PeerPlay's Google Group Error report is there to catch.
2. A country your track is available in
Closed testing tracks in Play Console have their own country and region availability. If a tester's Play Store country is not on that list, they can be fully opted in and still see that the app is not available. Check the track's country settings before you recruit, especially if you only picked your home country when you set it up.
A tester's Play Store country is tied to their Google account and payment profile, not to where they happen to be today, and Google limits how often it can be changed. Don't ask testers to switch it. Open your track to their country instead.
On PeerPlay, developers set Target Countries when they add an app, either All Countries or a list of country codes, and testers see it under Requirements Info before they request to test. Not Available in My Country is the report a tester sends when that still goes wrong.
3. A device your app supports
Your app's minimum Android version, required hardware features and supported screen types decide which phones can install it. A tester on an older Android version or a phone without a feature your manifest requires may find the app missing or marked as not compatible.
Know your own requirements before you recruit, and say them up front. PeerPlay's app form has a Supported Devices field, which defaults to All Android Devices and suggests entries like Android 8.0+ or Tablets. Testers also need a real Android phone with the Play Store. iPhones can't install a Play test, and PeerPlay doesn't allow emulators.
4. An account that can install apps
Not every Google account behaves like a personal Gmail account. Work and school accounts are managed by an administrator, who may limit or block the Play Store. A tester who signs in with their work account may not be able to install anything outside what their employer allows.
Children's accounts are another case. Google accounts have a minimum age that varies by country, and younger users usually have supervised accounts where a parent controls what can be installed. Don't build your 12 on accounts whose installs someone else has to approve. When in doubt, ask testers to use a personal Google account.
Check before you count
In the message where you share your opt-in link, ask two quick questions: which country is your Play Store set to, and which Android version is your phone on? It takes a tester ten seconds and saves you days.
Ask each tester to confirm that the install actually worked, not just that they joined. Then compare your opted-in count in Play Console with your list on day 1. If someone can't install, replace them that day rather than hoping it fixes itself, and keep a few testers above 12 for exactly this reason.
FAQ
Can testers from any country join a Google Play closed test?
Only if your closed track is available in their Play Store country. Check the track's country and region settings in Play Console before you recruit.
Why can't my tester install the app after opting in?
The usual causes are a different Google account in the Play Store, a country your track isn't available in, an unsupported device or Android version, or a managed work or school account.
Can I use work or school Google accounts as testers?
Sometimes, but an administrator may restrict the Play Store on those accounts. A personal Google account is the safer choice.
Do testers need a specific Android version?
They need a version your app supports. If your minimum Android version is higher than a tester's phone, they won't be able to install it.