If the state passed to the reducer is undefined, you must explicitly return the initial state. The initial state may not be undefined. If you don’t want to set a value for this reducer, you can use null instead of undefined.

store.js

import { configureStore } from "@reduxjs/toolkit";
import { apiSlice } from "../slices/apiSlice";
import  cartSliceReducer from "../slices/cartSlice.js";

const store = configureStore({
    reducer: {
        [apiSlice.reducerPath]: apiSlice.reducer,
        cart:cartSliceReducer,
    },
    middleware: (getDefaultMiddleware) => getDefaultMiddleware().concat
        (apiSlice.middleware),
    devTools: true
})

export default store;

at store.js:5:15
cartSlice.js

import { createSlice     } from "@reduxjs/toolkit";

const initialstate = localStorage.getItem('cart') ? JSON.parse(localStorage.getItem('cart')) :
 { cartItems: [] };
const cartSlice = createSlice({
    name: 'cart',
    initialstate,
    reducers: {

    }
})

export default cartSlice.reducer;