I suggest deleting all the attempted installations to begin with. If you haven't already, follow the Installation Guide for Android users, provided by Elena:
(If you're still having problems, let me know)
Install instructions
For Android users!
Go to Google Drive and tap on Gacha Neon.apk. The installation will start.
⚠ Attention
How to fix "app not installed" (Link)
Dellt dev, the author of Gacha Redux, told me how to solve the "can't install" bug. Thank you very much.
- Go to your device settings and tap Apps & Notifications (or Apps in older versions of Android).
- Tap the three dots in the upper-right corner.
- Tap Special access.
- Tap Install unknown apps.
- Tap Chrome (or whichever web browser you use)
- Move Allow from this source to the On position.