Database backup over sqlcmd call from batch file
Do this:

1.> Open a Command Prompt window and type:
cd\
dir sqlcmd.exe /s

Our Command Prompt window returned the following:
C:\Program Files\Microsoft SQL Server\100\Tools\Binn

Note: if no sqlcmd.exe is found something happened with your SQL install.
To get you up and running I'd simply download the utilities themselves here:

https://www.microsoft.com/en-us/download/details.aspx?id=36433

2.> From the Command Prompt window command menu Mark and highlight that path that was returned and press Enter to put it in the paste buffer.

3.> Edit the System path (not the user path) within Windows and paste the path you just found just after all the %SYSTEMROOT% path references and click OK. You may need to restart your server for this to take affect.