Gulp SintaxError : Unexpected identifier

I try to edit a Gulp app on my local server. I thought it could something with ES6 or version, but I tried so hard and nothing.

I always have this error:

/var/www/html/carros/gulpfile.js:11
let dev = true;
    ^^^
SyntaxError: Unexpected identifier
    at Module._compile (module.js:439:25)
    at Object.Module._extensions..js (module.js:474:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.require (module.js:364:17)
    at require (module.js:380:17)
    at Liftoff.handleArguments (/usr/local/lib/node_modules/gulp/bin/gulp.js:116:3)
    at Liftoff.<anonymous> (/usr/local/lib/node_modules/gulp/node_modules/liftoff/index.js:198:16)
    at module.exports (/usr/local/lib/node_modules/gulp/node_modules/liftoff/node_modules/flagged-respawn/index.js:17:3)
    at Liftoff.<anonymous> (/usr/local/lib/node_modules/gulp/node_modules/liftoff/index.js:190:9)

via luiza