An app that opens for its developer can still be unavailable to a tester. Treat the failed access attempt as a reproducible problem rather than assuming the link itself is broken.

Start with the exact failure

Ask for the full error, the opt-in URL, the Android version and whether the failure happens before joining, during installation or after opening the app. Request the affected Google email privately, not in a public forum. A login error inside your app is a different problem from Play Store access.

Check one layer at a time

Compare the package identifier in the link with the intended app.
Confirm the tester is using the Google account you authorized, including in the Play Store.
Inspect the selected tester list and the active release in the intended closed track.
Check Publishing overview for changes that still need action.
Check country availability and device compatibility.

Google's testing documentation explains that eligible testers must join through the test flow. Sending a store listing alone does not replace that step.

Retest without destroying useful evidence

Change one setting, note the time and ask the same tester to retry. Do not remove the whole group or recreate the app as a first response. If several people fail, compare their accounts and devices before deciding the problem is universal.

A useful support message is: “Build 8, closed track Alpha, access change published; joining works but installation fails on this device with this error.” That is much more actionable than “the app does not work.”