If you let your app to run in android 12, there is a new PendingIntent  mutability flag. If you don't want your PendingIntent  to be mutated, use
PendingIntent pendingIntent;
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.M) {
        pendingIntent = PendingIntent.getActivity(this,
                0, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT | PendingIntent.FLAG_IMMUTABLE);
    }else {
        pendingIntent = PendingIntent.getActivity(this,
                0, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT);
    }
If you want your PendingIntent  to be mutated use the following:
PendingIntent pendingIntent;
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.S) {
        pendingIntent = PendingIntent.getActivity(this,
                0, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT | PendingIntent.FLAG_MUTABLE);
    }else {
        pendingIntent = PendingIntent.getActivity(this,
                0, notificationIntent, PendingIntent.FLAG_UPDATE_CURRENT);
    }
In Google documentation says, Strongly consider using FLAG_IMMUTABLE, only use FLAG_MUTABLE if some functionality depends on the PendingIntent being mutable. The change should be straightforward.
Also, make sure you add the following work manager dependency if you are using AdMob 20.4.0 or lower in your app:
//Work Manager dependency
implementation 'androidx.work:work-runtime:2.7.1'
Note that currently work manager dependency version is 2.7.1. You can update the version to the latest one if you want.