错误:找不到模块'gulp-sass'

当我吞下肚,我得到了一个像下面的错误。 我该如何解决它?

module.js:339

throw err; ^ 

错误:找不到模块'gulpsass'

 at Function.Module._resolveFilename (module.js:337:15) at Function.Module._load (module.js:287:25) at Module.require (module.js:366:17) at require (module.js:385:17) at Object.<anonymous> (/Applications/XAMPP/xamppfiles/htdocs/flyscoot.com/gulpfile.js:2:12) at Module._compile (module.js:435:26) at Object.Module._extensions..js (module.js:442:10) at Module.load (module.js:356:32) at Function.Module._load (module.js:311:12) at Module.require (module.js:366:17)