If you npm install a Cordova/Capacitor plugin after npx cap add android then make sure you run npx cap update before running from Android Studio again.
If you make html, js, ts, css changes, run ionic build again and also run npx cap copy.
Or when in doubt, run npx cap sync as it does update+copy.

https://github.com/ionic-team/capacitor/issues/1213
https://stackoverflow.com/questions/23060038/cant-use-barcode-scanner-in-cordova-plugin-is-installed

1. init

  
npx @capacitor/cli create testbarcode  
npx cap add android  
npx cap sync  
npx cap copy  
npx cap open android  
  
ctrl+c exit

2. npm i phonegap-plugin-barcodescanner

  
npm i phonegap-plugin-barcodescanner --save  
npx cap sync  
npx cap copy

3. modify www/index.html

ADD

  
<button onclick="myFunction2()">Try it barcode</button>  
  
<script>  
    async function myFunction2() {  
      cordova.plugins.barcodeScanner.scan(  
        result => console.log(result),  
        err => console.error(err),  
        {  
          showTorchButton: true,  
          prompt: "Scan your code",  
          formats: "QR_CODE",  
          resultDisplayDuration: 0  
        }  
      );  
    }  
</script>  


  
npx cap sync  
npx cap copy  
  
android studio run

PS: use cordova.plugins NOT window.cordova.plugins