Targeting S+ (version 31 and above) requires that one of FLAG_IMMUTABLE or FLAG_MUTABLE be specified when creating a PendingIntent

[little-infinity]

After updating android target Sdk to 31. Before that it was working fine.
build.gradle

buildscript {
    ext {
        buildToolsVersion = "30.0.3"
        minSdkVersion = 28
        compileSdkVersion = 31
        targetSdkVersion = 31
        supportLibVersion = "30.0.0"
    }
    repositories {
        google()
        jcenter()
    }
    dependencies {
        classpath("com.android.tools.build:gradle:3.4.0")
    }
}

As soon as a USB device is connected following error gets thrown :

Targeting S+ (version 31 and above) requires that one of FLAG_IMMUTABLE or FLAG_MUTABLE be specified when creating a PendingIntent. Strongly consider using FLAG_IMMUTABLE, only use FLAG_MUTABLE if some functionality depends on the PendingIntent being mutable, e.g. if it needs to be used with inline replies or bubbles.

[little-infinity]

The cause of this error can be found in the RNSerialPortModule.java file.

method : requestUserPermission()

 private void requestUserPermission() {
    if(device == null)
      return;
    PendingIntent mPendingIntent = PendingIntent.getBroadcast(reactContext, 0 , new Intent(ACTION_USB_PERMISSION), 0);
    usbManager.requestPermission(device, mPendingIntent);
  }

Android API 31 onwards you need to set explicit FLAG while creating a PendingIntent https://developer.android.com/reference/android/app/PendingIntent#FLAG_MUTABLE

  private void requestUserPermission() {
    if (device == null) 
      return;
    PendingIntent mPendingIntent = PendingIntent.getBroadcast(reactContext, 0, new Intent(ACTION_USB_PERMISSION), 
    PendingIntent.FLAG_MUTABLE);
    usbManager.requestPermission(device, mPendingIntent);
  }

Till the time this fix is available you can patch it using patch-package :

1. Update requestUserPermission() in RNSerialPortModule.java:

  private void requestUserPermission() {
    if (device == null) 
      return;
    PendingIntent mPendingIntent = PendingIntent.getBroadcast(reactContext, 0, new Intent(ACTION_USB_PERMISSION), 
    PendingIntent.FLAG_MUTABLE);
    usbManager.requestPermission(device, mPendingIntent);
  }

2. Install patch-package:
   npm install patch-package -D
   or using yarn
   yarn add patch-package

3. add postinstall script to package.json:

"scripts": {
  "postinstall": "patch-package"
 }

4. Run patch-package to create a .patch file
   npx patch-package react-native-serialport or yarn patch-package react-native-serialport
   This will create .patch file with name react-native-serialport+1.3.1.patch - This can be committed in your project for the time being.
5. Apply your fix:
   npm install or yarn install

Hopefully, this works for you as well if someone is facing the same error.

[Goostavo]

I've fixed that problem on my branch:

https://github.com/goostavo/react-native-serialport