Heroku app crashing - everything works locally - heroku

Question

For my app I have my Postgres database deployed to AWS, server (Node and Express) and client (React) separately deployed to Heroku. My client application keeps crashing though and I'm unsure why. Everything works when run locally.
2020-05-01T17:18:33.292129+00:00 app[web.1]: > jepurdy#0.1.0 start /app
2020-05-01T17:18:33.292130+00:00 app[web.1]: > react-scripts start
2020-05-01T17:18:33.292130+00:00 app[web.1]:
2020-05-01T17:18:36.464174+00:00 app[web.1]: ℹ ｢wds｣: Project is running at http://172.18.97.46/
2020-05-01T17:18:36.464711+00:00 app[web.1]: ℹ ｢wds｣: webpack output is served from
2020-05-01T17:18:36.464811+00:00 app[web.1]: ℹ ｢wds｣: Content not from webpack is served from /app/public
2020-05-01T17:18:36.464902+00:00 app[web.1]: ℹ ｢wds｣: 404s will fallback to /
2020-05-01T17:18:36.465139+00:00 app[web.1]: Starting the development server...
2020-05-01T17:18:36.465140+00:00 app[web.1]:
2020-05-01T17:18:36.613427+00:00 heroku[web.1]: State changed from starting to crashed
2020-05-01T17:18:37.853487+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/" host=jepurdy.herokuapp.com request_id=01b14ed3-4b6c-44e1-a2c9-7cfbd728b7ad fwd="75.155.134.45" dyno= connect= service= status=503 bytes= protocol=https
2020-05-01T17:18:38.248004+00:00 heroku[router]: at=error code=H10 desc="App crashed" method=GET path="/favicon.ico" host=jepurdy.herokuapp.com request_id=a3519a2e-1ef9-49be-baa4-d4ed19f9d833 fwd="75.155.134.45" dyno= connect= service= status=503 bytes= protocol=https
Here's the package.json for the client:
{
"name": "jepurdy",
"version": "0.1.0",
"private": true,
"dependencies": {
"#material-ui/core": "^4.9.7",
"#testing-library/jest-dom": "^4.2.4",
"#testing-library/react": "^9.3.2",
"#testing-library/user-event": "^7.1.2",
"axios": "^0.19.2",
"node-sass": "^4.14.0",
"react": "^16.13.1",
"react-dom": "^16.13.1",
"react-router-dom": "^5.1.2",
"react-scripts": "3.4.1",
"react-spinners-css": "^1.1.7",
"socket.io": "^2.3.0",
"socket.io-client": "^2.3.0"
},
"scripts": {
"start": "react-scripts start",
"build": "react-scripts build",
"test": "react-scripts test",
"eject": "react-scripts eject"
},
"eslintConfig": {
"extends": "react-app"
},
"browserslist": {
"production": [
">0.2%",
"not dead",
"not op_mini all"
],
"development": [
"last 1 chrome version",
"last 1 firefox version",
"last 1 safari version"
]
},
"proxy": "http://localhost:8080"
}