I've removed a lot of the external code from this as I didn't have access to those. But this compiles now...

ItemList.swift

import SwiftUI
import ComposableArchitecture

enum ViewState {
    case initial
    case loading
    case success
    case failure(any Error)
}

struct ItemLight: Equatable, Identifiable {
    let id: String
    var name: String
}

struct MyError: Error {
}

@Reducer
struct ItemListReducer {
    @ObservableState
    struct State {
        var status: ViewState = .initial
        var items: [ItemLight] = []
        var hasNextPage = false
    }

    enum Action: ViewAction {
        case view(View)
        case delegate(Delegate)
        case fetchResponse(Result<[ItemLight], any Error>)

        @CasePathable
        enum View { // It is common practice to move view actions to their own enum like this... (And then TCA helps in the view also).
//            case fetchFeed
            case scrolledToLastItem // name the view action after what happened to trigger it.
            case onAppear
            case refresh
            case itemButtonTapped(ItemLight)
        }

        @CasePathable
        enum Delegate {
            case showItem(ItemLight)
        }
    }

    var body: some ReducerOf<Self> {
        Reduce<State, Action> { state, action in
            switch action {
            case .view(.itemButtonTapped(let item)):
                return .send(.delegate(.showItem(item)))

            case .view(.onAppear):
                state.status = .loading
                return fetchFeedEffect
// Don't send actions back into the reducder to perform shared logic. Extract it to a function possibly.
//                return .send(.fetchFeed)

// renamed action to reflect the view action that triggered it.
//            case .fetchFeed:
            case .view(.scrolledToLastItem):
                return fetchFeedEffect

            case .fetchResponse(.success(let items)):
                state.status = .success
                state.items.append(contentsOf: items)
                return .none

            case .fetchResponse(.failure(let error)):
                print(error.localizedDescription)
                return .none

            case .view(.refresh):
                return fetchFeedEffect

            case .delegate:
                return .none // delegate actions aren't used here. So return none for any of them
            }
        }
    }

    var fetchFeedEffect: Effect<Action> {
        .run { send in
            await send(.fetchResponse(.success([])))
        }
    }
}

@ViewAction(for: ItemListReducer.self) // this means you can just do `send(.someAction)`
struct ItemListView: View {
    @Environment(\.colorScheme) var colorScheme
    let store: StoreOf<ItemListReducer>

    init(store: StoreOf<ItemListReducer>) {
        self.store = store
    }

    var body: some View {
        switch store.status { // don't need to use `store.state.xxx` just access `store.xxx`.
        case .initial:
            Color.clear.onAppear {
                send(.onAppear)
            }
        case .loading:
            ProgressView()
        case .failure(let error):
            Text(error.localizedDescription)
                .refreshable {
                    await send(.refresh).finish()
                }
        case .success:
            if !store.items.isEmpty {
                ScrollView(.vertical) {
                    VStack {
                        LazyVStack {
                            ForEach(store.items) { item in // this is just one item. Call it `item` not `items`.
                                Button(action: {
                                    send(.itemButtonTapped(item))
                                }, label: {
                                    // ItemPreview(items: items) // Again... this preview displays a single `item`. Not multiple `items`.
                                    Text(item.name)
                                        .frame(height: 280)
                                        .frame(minHeight: 280)
                                })
                                .onAppear {
                                    if store.hasNextPage != false {
                                        if item == store.items.last { // again... singular `item`
                                            send(.scrolledToLastItem)
                                        }
                                    }
                                }
                            }
                        }
                        .refreshable {
                            await send(.refresh).finish()
                        }
                    }
                }
            } else {
                Text("No items available")
                    .refreshable {
                        await send(.refresh).finish()
                    }
            }
        }
    }
}

ParentReducer.swift

import SwiftUI
import ComposableArchitecture

@Reducer
struct ItemReducer {
    @ObservableState
    struct State {
        var item: ItemLight
    }
}

@Reducer
struct ParentReducer {
    @Reducer
    enum Path {
        case itemList(ItemListReducer)
        case item(ItemReducer)
    }

    @ObservableState
    struct State {
        var path = StackState<Path.State>()
        var itemList = ItemListReducer.State()
    }

    enum Action {
        case path(StackAction<Path.State, Path.Action>)
        case itemList(ItemListReducer.Action)
    }

    init() {}

    var body: some ReducerOf<Self> {
        Scope(state: \.itemList, action: \.itemList) {
            ItemListReducer()
        }
        Reduce<State, Action> { state, action in
            switch action {
                // ItemList
            case .itemList(.delegate(.showItem(let item))): // Only deal with the delegate actions here.
                state.path.append(.item(ItemReducer.State(item: item)))
                return .none

            case .itemList:
                return .none

            // Nested ItemList
            case .path(.element(id: _, action: .itemList(.delegate(.showItem(let item))))): // again, only deal with the delegate actions
                state.path.append(.item(ItemReducer.State(item: item)))
                return .none

                // Nested Item
//            case .path(.element(id: _, action: .item(.delegate(.showList))): // Again, this should be a delegate action.
//                state.path.append(.itemList(ItemListReducer.State()))
//                return .none
                
            case .path:
                return .none
            }
        }
        .forEach(\.path, action: \.path)
    }
}

struct ParentReducerView: View {
    @Bindable var store: StoreOf<ParentReducer>

    public init(store: StoreOf<ParentReducer>) {
        self.store = store
    }

    var body: some View {
        NavigationStack(path: $store.scope(state: \.path, action: \.path)) {
            ItemListView(store: store.scope(state: \.itemList, action: \.itemList))
        } destination: { store in
            switch store.case {
            case .itemList(let store):
                ItemListView(store: store)
            case .item(let store):
                ItemView(store: store)
            }
        }
    }
}

struct ItemView: View {
    let store: StoreOf<ItemReducer>

    var body: some View {
        Text(store.item.name)
    }
}

Some of the changes I made...

1. Move view actions into their own enum and use the ViewAction protocol available in TCA.
2. Rename the view actions to reflect what causes them to be triggered. Not what they should do.
3. Remove the use of store.state.xyz in favour of store.xyz.
4. Make sure to use singular names when dealing with a singular item (not items).
5. Extract shared logic into an effect that can be called. Sending actions back into a reducer is an anti pattern. (Except for delegate actions and responses from a .run effect.
6. Rename the delegate action to reflect what you want the delegate to actually do.
7. Rework the parent to only listen to delegate actions.