A: Make sure that Google confirmed your purchase and e-mailed Purchase Confirmation to your bound Google account (e-mail). Make sure your device has Google Play Store working and you can see your purchases there. Sometimes Google Play services misbehave (long lasting bug) and provide incorrect reply to the application's request to validate license.

There are 2 ways to try to fix this problem: 

1. Easy: 

• Reboot device
• Open Neutron from its page in Google Play Store app

2. Complex: 

• Uninstall Neutron (backup /NeutronMP with all settings but normally it should persist)
• Go to OS settings -> Apps -> find Google Play Store in the list -> clear cache and data -> find Google Play services -> clear cache and data -> reboot device
• Install and open Neutron from its page in Google Play Store app
• Access Neutron's settings -> Help -> Account -> wait until progress bar stops and red dot appears on GOOGLE entry

If these steps do not help send message to the technical support e-mail.

Note! Google Play license check does not work on Android devices without Google Play services installed. Earlier purchased applications will not be able to check license in Google Play and will behave as not purchased. If Google Play services can not be installed on such device to overcome this limitation and be able to use Neutron you could follow this FAQ answer.