根据您发布的内容，您很可能从未调用过spinner()函数，这意味着您从未设置适配器使其保持为空。
您还将R.layout.transaction_add_fragment传递给微调适配器，这可能不是您想要的。您传递到那里的布局资源应该是微调行的。
不要把它放在一个单独的函数中，只需把它添加到onViewCreated中，如下所示：

class AddTransaction: Fragment() {
    private var _binding: TransactionAddFragmentBinding? = null
    private val binding get() = _binding!!

    override fun onCreateView(
        inflater: LayoutInflater,
        container: ViewGroup?,
        savedInstanceState: Bundle?
    ): View? {
        _binding = TransactionAddFragmentBinding.inflate(inflater, container, false)
        return binding.root
    }

    override fun onViewCreated(view: View, savedInstanceState: Bundle?) {
        super.onViewCreated(view, savedInstanceState)
        
        // set the adapter here, and there is no need to make a separate "spinner"
        // class member since "binding" is already a class member that contains
        // the spinner
        binding.spinnerCategory.adapter = ArrayAdapter.createFromResource(
            requireContext(), R.array.categories, android.R.layout.simple_spinner_item)
    }
}