Closed testing requirement
New personal Google Play developer accounts must run a closed test before they can publish to production: at least 12 testers opted in when you apply, and opted in continuously for the preceding 14 days.
Get the number right, because the internet is full of the old one. The requirement launched at 20 testers and was reduced to 12. Articles, forum answers and video guides still say 20, and some say 20 for 20 days. The current figure is 12 testers over 14 continuous days.
Two words in that sentence do the damage. Opted in means each tester accepted the invitation and joined the test, not that you emailed them. Continuously means the count must hold across the whole window, so a tester who opts out on day 9 breaks your streak rather than shortening it. Watch the opt-in count in Play Console rather than your own list, and keep a buffer above 12.
Once the window is satisfied, production access is an application, not an automatic unlock. You describe how you tested and what you learned, and Google reviews it.
The requirement applies to personal accounts, which is the type most solo developers create. Organisation accounts, which need verifiable business details, are not subject to it. That difference is worth knowing before you sign up, because account type is not trivially changed afterwards.
See also: track, Play Console, staged rollout.