Package appears to be invalid

If the screen says App not installed as package appears to be invalid, the phone will not take the file as an APK. Installation does not start, because the package does not open.

The same sentence comes from more than one cause. The order is: a half file, a wrong ending, an old signature, and Android 14 or 15 rejecting an old package.

• The file stopped halfway, or it was compressed when sent from WhatsApp. Delete the old copy and download it again. Do not install a file that has not finished.
• If the name is still .apk.apk, .zip, or .xapk, this sentence appears. A single APK must end in .apk. Renaming an XAPK does not make it an APK.
• If another signature of the same app is already on the phone, the package is treated as invalid. On Samsung and Xiaomi the new one does not replace the old until you remove it. The data goes away.
• Android 14 and 15 treat some old packages as invalid. The package has to be built again with a current tool. Making the phone older does not fix it.
• On a Xiaomi, if the file sits in Downloads under another name, the installer will not open it. Tap the original name from the file manager.

The same words also appear when the list inside the file is broken. If you edited the inside by hand, build the file again.