An APK that refuses to install is doing you a favour until you know why. Parse errors, Play Protect blocks, and “app not installed” toasts have different causes. This walk-through matches the troubleshooting section on the install guide with a bit more room to breathe.
Corrupt or truncated files
If the download bar never finished, delete the file. Do not join two partials from two websites. Retry on Wi-Fi you control. A parse error after a complete download often means the package was never a valid Android package to begin with.
Blocked for a reason
Manufacturer layers and Play Protect exist because sideload is a common malware path. Searching for a way to ignore the warning is how a sports curiosity becomes a device incident. Use a Play Store client instead.
Crashes after a “successful” install
Clear storage once. If the app still dies on open, uninstall it. Crash loops plus unexpected permission requests are a pair you should not debug past. See also the FAQ on updates and buffering.