You don’t typically reference a script in the node_modules folder because that doesn’t get shipped when you build the app. That is probably your issue because Capacitor isn’t going to copy the node_modules folder to the device.
Might be good to read up on Chart.js Integration.
You can also debug your app running on the device in DevTools to see any errors.
- Android: Go to chrome://inspect/#devices in Chrome
- iOS: How to Debug Websites on iPhone Safari | BrowserStack