Per a forum report from MysticTempest:

On my Ubuntu Mate 18.04 Linux system I was having issues with SMAPI 3.0 on SDV v1.4.
I was receiving this error about SMAPI detecting an Android system instead of desktop Linux.

[game] The game failed to launch: System.InvalidOperationException: Unknown target platform 'Android'.

After some digging, I found in "SMAPI.Toolkit.xml" some comments about how it was checking for Android. It searches the system for the command, "getprop".
Well, on my system "getprop" happens to be part of a package called "libhybris-utils".
It seems to provide some extra support for Ubuntu Touch, & other android systems.

I didn't have any use for it, so I uninstalled it; and afterwards SMAPI was able to start correctly.
So, I figured I'd post the info in case someone else comes across this.

Fixed in develop for the upcoming SMAPI 3.0.2.