Trouble installing SwiftKey via APKPure

I don't have a Google account and use APKPure (and F-Droid) to get most apps I need. Now, there are a few apps which, after download, refuse to install - they give me the "App not installed" error when I try to install them from within the APKPure interface. Right now I'm seeing this with the SwiftKey Keyboard application: I already use it, and I'm trying to install an upgrade of it but getting that error. * How can I determine why I'm getting the "App not installed" message? * Should I try installing in "Safe Mode"? I've heard about it but don't yet know how to get into it. * If I want to try and install manually - where do I find the APK file(s) for my app? Additional informations: * I'm using a Xiaomi Redmi 3S with Android version 6.0.1. * SwiftKey is already installed and is in use; but just to be sure, I replaced it with another keyboard as the chose/active keyboard app and the results are the same. * The APK file _seems_ to be this one (assuming I get the same file from my desktop and from my APKPure app on my Android device). * $ apksigner verify SwiftKey\ Keyboard_v6.6.9.32_apkpure.com.apk WARNING: META-INF/DEPENDENCIES not protected by signature. Unauthorized modifications to this JAR entry will not be detected. Delete or move the entry outside of META-INF/.