APK will not install

If the phone says App not installed or There was a problem parsing the package, the APK will not install. Those two sentences are two names for the same stop: the file does not open, or the phone will not take it.

Do not change everything at once. Check the cause in order.

• If the file stopped halfway, installation does not start. Delete that copy, open the link again, and install only after the download finishes.
• If the name is .apk.apk or still .zip, the phone cannot read the package. The ending should be a single .apk.
• Unknown sources may be off. Samsung: Settings, Security and privacy, Install unknown apps, then allow the app that opens the file. Xiaomi: Settings, Apps, Unknown sources, then that same app.
• If the same app is already installed with a different signature, the new one will not go on top. Remove the old one first. The data inside goes away.
• An APK signed with another key will not install over an app from Play. That is not a file cut off on the way.

If parsing the package stays on the screen, the file is not an APK or it was cut off. Do not save it under a new name. Download it again.