Trouble installing SwiftKey via APKPure
Following @Izzy's patient advice in the comments, I got impatient and finally resolved this.

The reason for "App not installed" in OP's case

The pre-installed version of the SwiftKey Keyboard app had some sort of incompatibility with more recent versions - a "signature mismatch" - preventing an upgrade from the former to the latter. 

Should I try installing in "Safe Mode"?

Not relevant in this case.

So how do I upgrade SwiftKey?

Uninstall SwiftKey (just the main package; there may be auxiliary packages) and install the new version.### How to determine why you get "App not installed" generally

It's tricky. What you can do is have a look at the "logs":


Install a log viewer app like CatLog
Install something like Termux (or an SSH server, then SSH into your phone)
In a terminal, run adb logcat
Try installing SwiftKey
Have a look at the what you get in the log


You need to redirect the log output into a text file if you can't scroll your terminal well enough.