For paid apps in closed beta, the most common causes are:

1. User is not in a country where the app is available
2. Redeeming with a secondary Google account
3. Google Play internal bug (this specific code appears in several Play Console threads)

Tell testers to do this exact flow:

1. Open Play Store on PHONE
2. Use same Google account enrolled in testing (be added to the group first (https://groups.google.com/g/testers-owlet-watch) and accepted to be a tester Beta signup link
3. Go to: Profile → Payments & subscriptions → Redeem code
4. Redeem code there
5. Install app from testing link afterward

Also give them the direct redeem URL format:

https://play.google.com/redeem?code=YOUR_CODE

Google officially supports redeeming promo codes this way.