The instance member 'params' can't be accessed in an initializer

You can't access params before you've initialized the object. To fix your example, move your myTest initialization into a constructor.

Also, I don't believe you should have a period before [comLevel].

class LevelUp extends GetxController {
  Map<String, String> params = Get.arguments;
  String myTest;
  
  LevelUp() {
    myTest = params[comLevel];
  }
}