If the Licensing validation fails, it won't allow to users to open the App and shows the message "Please purchase the app", for remediation:  

Please make sure that you always have the mail ID, which you used for the purchase of Voice PRO in your device google accounts list. For certain devices, If you have more than 2 mail IDs, it is necessary to have the purchase mail ID as default/primary mail ID, so that it can easily validate. (Please refer the image for Primary mail ID)

Kindly Note: The App itself validates the license every-time you use, so please make sure you are connected with the internet (Data/Wifi) when you open the app.

If you still face issues on licensing or validation you can always reach us at support@voicepro.it / Mobihelp (Online chat option from the Voice PRO app itself). Your query will be addressed with highest priority.