Running system mono/msbuild, ERROR: Invalid SDK MacOSX.sdk, not found in /Library/Developer/CommandLineTools/Platforms/MacOSX.platform/Developer/SDKs code example

Example: Running system mono/msbuild, ERROR: Invalid SDK MacOSX.sdk, not found in /Library/Developer/CommandLineTools/Platforms/MacOSX.platform/Developer/SDKs

Turns out there’s a setting 
in Xcode’s preferences
under Locations which needs to be turned on. 
The part in the image called ‘Command Line Tools:’
was blank, so I toggled on the only option available.
I tried creating a new project and it worked.