I created a FireMonkey app in Delphi 10.1 (Berlin) (Win10, 64 bit). The app compiles and runs fine as 32-bit Windows, 64-bit Windows, and Android (running on an external device). I'm using no nonstandard components, though it does include an sqlite3 database.
I then switched to a MacMini I just recently set up for cross platform development (via Embarcadero's docwiki directions). It's running OS X High Sierra with Delphi 10.1 on a VirtualBox with Win 8.1 (64 bit). I set up PASServer, etc. for OS X development. I can compile and run the Windows version of the app here. I cannot, however, add OS X as a target platform. This is the problem I'm hoping you can help me with.
Here's what else I tried:
If I create a new app from scratch in this machine, I can compile and run it on OS X (so the issue is not me creating a bad connection profile or not having added OS X as a potential target platform).
If I take a simple, different (32-bit app) I created elsewhere I recreate the issue--I can run the app on the Windows VirtualBox but can't add OS X as a target platform.
Open the project's .dproj file in a text editor and make sure OSX is present and enabled in the <Platforms> section, eg:
<Platforms>
...
<Platform value="OSX32">True</Platform>
...
</Platforms>
That governs whether "Add Platform" will let you add OSX to the project or not.
Also see "Add Platform" is disabled on Embarcadero's forums.
Alternatively, just create a new project from scratch and add your existing source files to it.