App not installed" on Android: causes and fixes

One message, several causes

Android's package installer shows App not installed for any failure and hides the reason. The causes, roughly in order of how often they occur:

1. Signature conflict. A copy of the app signed with a different key is already installed, usually the Play version versus a build from another source, or vice versa. Fix: uninstall the existing app, then install the file. If you want to keep the Play version, you cannot install the other one.
2. Downgrade. The file's version code is lower than the installed one. Fix: uninstall first. See how to downgrade an app.
3. Not enough storage. Installation needs roughly twice the package size free. Fix: free space, then retry.
4. Corrupt or incomplete download. Common with large files over a mobile connection. Fix: compare the size with the one on the version page and download again.
5. Wrong architecture or Android version. A 64-bit-only build on a 32-bit phone, or a package requiring a newer Android. Fix: pick a version whose Requires Android fits, or the APK build instead of the XAPK.
6. Split bundle installed with the wrong tool. A base APK from an XAPK installed alone is missing its splits. Fix: install the whole bundle with an XAPK installer.

Finding the real reason

The installer's log carries the actual error code. Without a computer, the free Package Installer alternatives and Split APKs Installer show it in their result screen. With a computer, adb install file.apk prints codes such as INSTALL_FAILED_UPDATE_INCOMPATIBLE (signature), INSTALL_FAILED_VERSION_DOWNGRADE, INSTALL_FAILED_INSUFFICIENT_STORAGE or INSTALL_FAILED_NO_MATCHING_ABIS (architecture).

Manufacturer-specific traps

• Xiaomi / MIUI: the built-in "installer" security scan sometimes reports a failure for packages it cannot verify. Disable it in Security › Settings, or install with a file manager.
• Samsung: Auto Blocker on One UI 6 and later blocks installs from anything except the Play Store and Galaxy Store until you turn it off in Settings › Security and privacy.
• Android 13 and later: apps installed from a browser may need Allow restricted settings before they can install packages themselves.

If it still fails

Reboot the phone, clear the package installer's storage (Settings › Apps › Show system › Package installer › Storage › Clear), and try once more. If the same file installs on another device, the problem is local to yours; if it fails everywhere, the file is damaged.