I would like to show you how to fix the problem to build MonoGame sprite font:
Error: Processor 'FontDescriptionProcessor' had unexpected failure! *.spritefont
Processor 'FontDescriptionProcessor' had unexpected failure!
System.DllNotFoundException: Unable to load DLL 'freetype6.dll': The specified module could not be found. (Exception from HRESULT: 0x8007007E)
at ShartpFont.FT.FT_Init_FreeType(IntPtr& alibrary)
It meanst that you are missing the C++ 2012 SDK.
To resolve the problem install VS2012 Visual C++ Redist from Microsoft: https://www.microsoft.com/en-us/download/details.aspx?id=30679