Hi there! I understand you're having trouble logging into the Stockpile app using your fingerprint on your Kindle device running Android 9. Let's try to get that sorted out for you.

To start, can you tell me if the fingerprint login was working before and suddenly stopped, or is this the first time you're trying to use it? Also, have you tried any troubleshooting steps so far?

Meanwhile, here's a simple step-by-step approach we can try:

1. Check App Version: Make sure your Stockpile app is updated to the latest version. Sometimes, fingerprint login issues can occur if the app is outdated.

2. Device Fingerprint Settings: Verify that your Kindle's fingerprint sensor is correctly set up with your fingerprint registered in the device settings. You can review this in the Security section of your device settings.

3. App Permissions: Ensure that Stockpile has the necessary permissions to access biometric features. Go to Settings > Apps > Stockpile > Permissions, and confirm biometric or fingerprint permissions are allowed.

4. Disable VPN: If you use a VPN on your Kindle, temporarily disable it, as it can interfere with app authentication.

5. Log In with Username and Password: Try logging in manually with your username and password first to confirm your credentials are working.

6. Re-enable Fingerprint Login: After successfully logging in, check if there's an option within the Stockpile app to enable or re-enable fingerprint login and set it up again.

If after these steps fingerprint login still doesn't work, please let me know. There may be a need to clear app cache or reinstall the app, or we might consider other less common issues.

Are you comfortable trying these steps? And please let me know how far you can get or if you encounter any difficulties along the way!