Can't install F-Droid on Sonim XP3 Plus flip phone

I download the app from the phone’s browser. It finishes fairly quickly and appears in my downloads app. However, when I click on it says, “App not installed. F-Droid couldn’t be installed on your phone.”

We’d need some sort of logcat log…

How do I find that?

Via ADB, via an USB connected PC

Search the internet on how to