Cannot find module "morgan" and but I see morgan exists as dependencies in my package.json file

I am trying to open an app with npm start, but keep getting the error message

Error: cannot find module 'morgan'

I looked up some advices on github or stackoverflow, tried these commands:

npm install morgan

npm install --save morgan

npm link morgan

npm install mongoose-morgan

None of them works. And I do see morgan 1.10.0 in the dependencies of my package.json file.

Anyone knows what's happening?