Full Error :

ionic $ module.js:338
    throw err;
          ^
Error: Cannot find module 'bower'
    at Function.Module._resolveFilename (module.js:336:15)
    at Function.Module._load (module.js:278:25)
    at Module.require (module.js:365:17)
    at require (module.js:384:17)
    at Object. (/Users/jaswantdhayal/Desktop/Projects/Purpose it all/purposit/gulpfile.js:3:13)
    at Module._compile (module.js:460:26)
    at Object.Module._extensions..js (module.js:478:10)
    at Module.load (module.js:355:32)
    at Function.Module._load (module.js:310:12)
    at Module.require (module.js:365:17)

Solution : => Go to the project directory

$rm -rf node_modules
$npm install