How to change location of index.js - npm

Question

I'm trying to re arrange a project repo from this:
public/
src/
|
|---- index.js
package.json
To this:
public/
frontend/
|
|---- src/
|
|---- index.js
package.json
When I rearranged, npm start would fail with this message:
Could not find a required file.
Name: index.js
Searched in: /Users/sahandzarrinkoub/Documents/Programming/DH2642/Sentify/repo/Sentify/Sentify/src
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! Sentify#0.1.0 start: `react-scripts start`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the Sentify#0.1.0 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
So I went into the package.json file and added this:
"main": "frontend/src/index.js",
What's strange is that nothing changes. npm tells me that it's still looking in the same directory for index.js.
Could not find a required file.
Name: index.js
Searched in: /Users/sahandzarrinkoub/Documents/Programming/DH2642/Sentify/repo/Sentify/Sentify/src
npm ERR! code ELIFECYCLE
npm ERR! errno 1
npm ERR! sentify#0.1.0 start: `react-scripts start`
npm ERR! Exit status 1
npm ERR!
npm ERR! Failed at the sentify#0.1.0 start script.
npm ERR! This is probably not a problem with npm. There is likely additional logging output above.
How can I successfully rearrange my project into this organisation? How can I tell npm where to look for index.js?
I also tried:
"main": "/Users/sahandzarrinkoub/Documents/Programming/DH2642/Sentify/repo/Sentify/Sentify/frontend/src/index.js",