If Python is installed using a package manager, it should already be on your path.
But if installed using 'msi/exe', then add "C:\Python27" to Environment-User-variable PATH (or the location where it is installed)
Node-gyp requires Python 2.x and cannot use Python 3.x

If path is required only at the specific directory, then try below:
npm config set python C:\Python27\python.exe