Não consigo executar o comando npm start apresenta o erro abaixo.

C:\Users\johnl\Documents\Projetos
> nodemon index.js

[nodemon] 2.0.12
[nodemon] to restart at any time, enter `rs`
[nodemon] watching path(s): *.*
[nodemon] watching extensions: js,mjs,json
[nodemon] starting `node index.js`
internal/modules/cjs/loader.js:892
  throw err;
  ^

Error: Cannot find module 'express'
Require stack:
- C:\Users\johnl\Documents\Projetos\index.js
    at Function.Module._resolveFilename (internal/modules/cjs/loader.js:889:15)
    at Function.Module._load (internal/modules/cjs/loader.js:745:27)
    at Module.require (internal/modules/cjs/loader.js:961:19)
    at require (internal/modules/cjs/helpers.js:92:18)
    at Object.<anonymous> (C:\Users\johnl\Documents\Projetos\index.js:1:17)
    at Module._compile (internal/modules/cjs/loader.js:1072:14)
    at Object.Module._extensions..js (internal/modules/cjs/loader.js:1101:10)
    at Module.load (internal/modules/cjs/loader.js:937:32)
    at Function.Module._load (internal/modules/cjs/loader.js:778:12)
    at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:76:12) {
  code: 'MODULE_NOT_FOUND',
  requireStack: [ 'C:\\Users\\johnl\\Documents\\Projetos\\index.js' ]
}
[nodemon] app crashed - waiting for file changes before starting...