Hi,
Please follow the steps below to activate the Pro app:
0. Open device Settings > Accounts > Google > leave only ONE Google account there:
1. Open Google Play Store with the same account:
2. Tap the menu in the upper left, My Apps & Games, tap 'Library'
3. Select the app, tap Install or Enable.
4. Open Aqua Mail > add as an account in there
Thank you,
Nadia