The package appears to be invalid

"The package appears to be invalid" means the phone will not accept the file as an APK. Installation does not start, because the package does not open.

• The file stopped halfway, or another app compressed it while sending. Download it again.
• If the name is .apk.apk or still .zip, the ending should be a single .apk.
• Android 14 and 15 treat some old packages as invalid. Build the package again with a current tool.

The same words also appear when the list inside the file is broken.