On a Windows system, the commands function instantiates a 32-bit command process, so any command with 64-bit dependencies may not be recognized or run properly. There are numerous reasons, but in many cases it may be triggered by .net Classes, Snapins, Modules, or even COM Objects that are 64-bit dependent.
%windir%\sysnative\cmd.exe /C your_command
%windir%\sysnative\cmd.exe /C wbadmin enable backup
%windir%\sysnative\windowspowershell\v1.0\powershell.exe wbadmin enable backup