Only Macintosh shown on Startup Manager after Windows 10 32-bit Boot Camp install
32-bit versions of Windows 10 are not supported on any Mac model. You need to install the 64-bit version.

From Apple KB: How to install Windows using Boot Camp


  Before you begin, make sure you have everything you need:
  
  
  An Intel-based Mac
  A Microsoft Windows installation media or disk image (ISO) containing a 64-bit version of Microsoft Windows 8 or later
  


See this list for which Mac models support Windows 10.