Ionic setup sass error

Hi, after I installed gulp, I have tried to setup sass but I receive following err:

…\chapter7\sass\node_modules\node-sass>if not defined npm_config_node_gyp (node “C:\Program Files\nodejs\node_modules\npm\bin\node-gyp-bin\…\node_modules\node-gyp\bin\node-gyp.js” rebuild ) else (node “” rebuild )
(node:5500) DeprecationWarning: child_process: options.customFds option is deprecated. Use options.stdio instead.
gyp ERR! configure error
gyp ERR! stack Error: Can’t find Python executable “python”, you can set the PYTHON env variable.
gyp ERR! stack at failNoPython (C:\Program Files\nodejs\node_modules\npm\node_modules\node-gyp\lib\configure.js:449:14)
gyp ERR! stack at C:\Program Files\nodejs\node_modules\npm\node_modules\node-gyp\lib\configure.js:404:11
gyp ERR! stack at C:\Program Files\nodejs\node_modules\npm\node_modules\graceful-fs\polyfills.js:264:29
gyp ERR! stack at FSReqWrap.oncomplete (fs.js:123:15)
gyp ERR! System Windows_NT 10.0.10240
gyp ERR! command “C:\Program Files\nodejs\node.exe” “C:\Program Files\nodejs\node_modules\npm\node_modules\node-gyp\bin\node-gyp.js” "rebuild"
gyp ERR! cwd E:\Projects\MobileApp\ionic\projects\TestPrograms\chapter7\sass\node_modules\node-sass
gyp ERR! node -v v6.9.4
gyp ERR! node-gyp -v v3.4.0
gyp ERR! not ok
Build failed

What can I do? any Idea…
please help me about this issue.