If the APK failed to install with "package appears to be invalid," that was a real bug, not something wrong on your end.
Android requires every app to be cryptographically signed before it can be installed, this is how the system knows an update genuinely comes from the same developer as the original. The build configuration for PurgeCore Mobile had no signing setup at all. Gradle quietly produced a technically valid but completely unsigned APK, which some emulators tolerated but which real Android devices correctly reject outright.
Every release APK is now signed, with the same key each time. That second part matters as much as the first: a new version can now install as a clean update over the previous one, rather than requiring an uninstall first. This isn't a verified commercial certificate yet, so Android will still show it as coming from an unknown developer, the same "unknown publisher" situation PurgeCore for Windows is in until a real signing certificate exists.
If you tried installing an earlier version and it failed, this is exactly why, v1.0.4 and onward should install cleanly.
Free, v1.0.4, not yet on Google Play.
Download the APKSee the PurgeCore Mobile page for the full feature list and FAQ.