If the Play Store asks you to buy the application when you have already done so in the past with the same Gmail address, it’s probably because the data in the Play Store app is corrupted. To repair that:
- Open your device’s Settings appÂ
.
- Tap Apps & notifications
See all apps.
- Scroll down and tapGoogle Play StoreÂ
.
- Tap StorageÂ
 Clear Cache.
- Next, tap Clear data.
- Re-open the Play Store & try your download again.
If this doesn’t solve the problem, try following Google’s advice by clicking on this link.