If you downloaded 'Gran Saga Idle: KNIGHTS X KNIGHTS' through the Google Play Store,

 You can set a payment password to prevent unintentional purchases.

 How to set a Google Play Store payment password

  - 1. Launch the Google Play Store and tap your profile picture on the right side of the search bar.

  - 2. Select Settings > User controls > Require authentication for purchases

  - 3. Select "Authenticate every time you make a purchase through Google Play on this device

  - 4. Select "Set a payment password for your Google account