Hi! I see you're having trouble opening Melon Sandbox on your Kindle running Android 9, and it’s not even showing up in your list of installed apps. Let’s get to the bottom of this together.

First, a couple of quick questions to better understand:
- Did you download Melon Sandbox from the official Google Play Store or from another source?
- After downloading, did you get any error messages or prompts during installation?
- Have you checked the "Apps" or "Apps & Notifications" section in your Kindle’s Settings to see if the app is listed there, maybe under a different name?

While you check those, here’s a step-by-step that usually helps in similar cases:

1. Verify Installation:
- Go to Settings > Apps & Notifications > See all apps.
- Scroll to see if Melon Sandbox or any app with a similar name is listed.
- If it’s listed but you can’t open it from there, tap it and try "Force Stop" then "Open."

2. Reinstall the App:
- If you don’t see it, the installation might not have completed properly.
- Go to the Play Store (or the source you used) and try to install it again.
- Ensure your device has enough storage space and a stable internet connection during installation.

3. Check for Device Compatibility:
- Sometimes an app might not be fully compatible with Kindle devices or older Android versions (like Android 9).
- You could check the app page on Google Play for any notes about requirements.

4. Clear Cache and Data for the Package Installer:
- Go to Settings > Apps > Show system apps (you may need to tap the three dots to enable this).
- Find "Package Installer" or "Google Play Store."
- Clear Cache and Clear Data.
- Try reinstalling the app.

5. Look for Downloaded APK Files:
- If you downloaded the app as an APK from outside the Play Store, it might require manual installation.
- Go to your "Files" app, locate the APK file, and tap it to install.
- Make sure you’ve enabled "Install from Unknown Sources" in the security settings.

If you've tried reinstalling or if the app shows any error during launch or installation, let me know. I can guide you to more specific fixes or check if there are known issues with the app on Kindle devices.

One last tip: sometimes, uninstalling updates for the "Files" app (the file manager) has helped users recover visibility or functionality for certain apps, in case the Kindle's file system integration is causing issues.

I’ll be here to help as you try these steps! Are you able to check these and tell me what you find?