1. Open the 'Play Store app'> Tap the 'User Icon (Menu)' in the upper right > Tap on 'Settings'> 'User Controls'> 'Require authentication when purchasing'> Select the desired authentication method

ᆞ Authenticate every time you make a purchase through Google Play on this device: Authentication is required every time you make a payment on Google Play.
ᆞEvery 30 minutes: When making a purchase on Google Play, a passcode is requested every 30 minutes. Once you enter your password, you will not need to enter it for the next 30 minutes.
ᆞ No verification: Payment is done on Google Play without any verification steps. (Not recommended)

[Go to Google Play Help]