Why is heroku not connecting express and giving an error?

Question

Node.js

Why when heroku writes that there is no express module even though I installed it.
The error itself (if it helps):

2020-06-04T19:29:03.065465+00:00 app[web.1]: Error: Cannot find module 'express'
2020-06-04T19:29:03.065465+00:00 app[web.1]: Require stack:
2020-06-04T19:29:03.065465+00:00 app[web.1]: - /app/index.js
2020-06-04T19:29:03.065473+00:00 app[web.1]: at Function.Module._resolveFilename (internal/modules/cjs/loader.js:966:15)
2020-06-04T19:29:03.065473+00:00 app[web.1]: at Function.Module._load (internal/modules/cjs/loader.js:842:27)
2020-06-04T19:29:03.065479+00:00 app[web.1]: at Module.require (internal/modules/cjs/loader.js:1026:19)
2020-06-04T19:29:03.065479+00:00 app[web.1]: at require (internal/modules/cjs/helpers.js:72:18)
2020-06-04T19:29:03.065480+00:00 app[web.1]: at Object.<anonymous> (/app/index.js:1:11)
2020-06-04T19:29:03.065480+00:00 app[web.1]: at Module._compile (internal/modules/cjs/loader.js:1138:30)
2020-06-04T19:29:03.065480+00:00 app[web.1]: at Object.Module._extensions..js (internal/modules/cjs/loader.js:1158:10)
2020-06-04T19:29:03.065480+00:00 app[web.1]: at Module.load (internal/modules/cjs/loader.js:986:32)
2020-06-04T19:29:03.065481+00:00 app[web.1]: at Function.Module._load (internal/modules/cjs/loader.js:879:14)
2020-06-04T19:29:03.065486+00:00 app[web.1]: at Function.executeUserEntryPoint [as runMain] (internal/modules/run_main.js:71:12) {
2020-06-04T19:29:03.065491+00:00 app[web.1]: code: 'MODULE_NOT_FOUND',
2020-06-04T19:29:03.065491+00:00 app[web.1]: requireStack: [ '/app/index.js' ]
2020-06-04T19:29:03.065492+00:00 app[web.1]: }

and link to github: https://github.com/Denisca2288/testing