The error is clear:

The argument type 'CarouselController' can't be assigned to the parameter type 'CarouselSliderController?'

Since in your code, you are using a CarouselSlider:

CarouselSlider(
        items: [

You should be passing in a CarouselSliderController.

But you were passing in a CarouselController — which may be used for a CarouselView.

So, use a CarouselSliderController with your code:

class OnboardingPage extends StatefulWidget {
  const OnboardingPage({super.key});

  @override
  State<OnboardingPage> createState() => _OnboardingPageState();
}

class _OnboardingPageState extends State<OnboardingPage> {
  int currentIndex = 0;
  CarouselSliderController carouselController = CarouselSliderController();

  @override
  Widget build(BuildContext context) {
    return Scaffold(
        backgroundColor: lightBackgroundColor,
        body: Center(
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: [
              CarouselSlider(
                items: [
                  Image.asset(
                    'assets/img_onboarding1.png',
                    height: 331,
                  ),
                  Image.asset(
                    'assets/img_onboarding2.png',
                    height: 331,
                  ),
                  Image.asset(
                    'assets/img_onboarding3.png',
                    height: 331,
                  ),
                ],
                options: CarouselOptions(
                  height: 331,
                  viewportFraction: 1,
                  enableInfiniteScroll: false,
                  onPageChanged: (index, reason) {
                    setState(() {
                      currentIndex = index;
                    });
                  },
                ),
                carouselController: carouselController,
              ),
            ],
          ),
        ));
  }
}