linux下离线安装了pm2之后，执行pm2报错

使用的是离线方式的安装，pm2-4.5.1.tar.gz
安装在了root文件夹下，
然后执行了 In -s /root/pm2/bin/pm2 /usr/local/bin/pm2后，
执行pm2 -v，报了以下错误：

internal/modules/cjs/loader.js:969
throw err;
^

Error: Cannot find module 'debug'
Require stack:

/root/pm2/constants.js /root/pm2/lib/binaries/CLI.js /root/pm2/bin/pm2
at Function.Module._resolveFilename (internal/modules/cjs/loader.js:966:15)
at Function.Module._load (internal/modules/cjs/loader.js:842:27)
at Module.require (internal/modules/cjs/loader.js:1026:19)
at require (internal/modules/cjs/helpers.js:72:18)
at Object.<anonymous> (/root/pm2/constants.js:7:14)
at Module._compile (internal/modules/cjs/loader.js:1138:30)
at Object.Module._extensions..js (internal/modules/cjs/loader.js:1158:10)
at Module.load (internal/modules/cjs/loader.js:986:32)
at Function.Module._load (internal/modules/cjs/loader.js:879:14)
at Module.require (internal/modules/cjs/loader.js:1026:19) {
code: 'MODULE_NOT_FOUND',
requireStack: [
'/root/pm2/constants.js',
'/root/pm2/lib/binaries/CLI.js',
'/root/pm2/bin/pm2'
]

}

请问错误要怎么解决？