确保你安装了grunt模块 npm install grunt

@kingapple grunt-cli源码中的方法

try {
  gruntpath = resolve('grunt', {basedir: basedir});
} catch (ex) {
  gruntpath = findup('lib/grunt.js');
  // No grunt install found!
  if (!gruntpath) {
    if (options.version) { process.exit(); }
    if (options.help) { info.help(); }
    info.fatal('Unable to find local grunt.', 99);
  }
}