The problem is in-app purchasing can't be processed due to Google Play library version is low.
Please see the following guide to resolve the issue.
Update Flexcil app to the latest version
Go to the Google Play Store and update Flexcil app to the latest version
Update the Google Play Store app to the latest version
Open the Google Play Store app and check if the app is the latest version.
You can see the version by pressing the Account at the top right and going Settings>About>Play Store version.
If the Google Play Store app is not the latest version, press Update Play Store and updates the app.
Clear cache and data
- Restart your device.
- Go Settings>Apps
- Search Google Play Store on the list from the panel.
- Select Storage in the Usage section.
- Press Clear data and Clear cache at the bottom.
Sign out and sign in Google account
- Go Settings>Accounts
- Sign out of your all accounts.
- Open the Google Play Store app and sign in with your Google Play account.