Select Menu → License → I already have a license:
Then click on Recover purchase from Google Play.
Enter the email address you used when purchasing this license, and your personal data
Activation error
If any error messages appear, try to activate your license using a different network connection.
If you continue to encounter problems, a detailed diagnostic is required.
Submit a written request to our technical support service. Please attach the error screenshot to your request; indicate your Google Play order number (GPA-...) and the Google account address (your_email@gmail.com) you used to make the purchase.