question

Because each PlayFab API has some form of security that needs to be passed in the header of each request. For Client API requests, this security is a session ticket from a login request. Or it could be your title secret key if you are making server API requests. If you want to call the Client API, you need to call it in the OnLoginSucess function. About the QuickStart of login requests, you can refer to the Unity3D quickstart. We have check this API, it works fine, so please check the Login functions in your clients.

using PlayFab;
using PlayFab.ClientModels;
using UnityEngine;

public class PlayFabLogin : MonoBehaviour
{
    public void Start()
    {
        if (string.IsNullOrEmpty(PlayFabSettings.staticSettings.TitleId)){
            PlayFabSettings.staticSettings.TitleId = "[YourTitleID]"; // Please change this value to your own titleId from PlayFab Game Manager
        }
        var request = new LoginWithCustomIDRequest { CustomId = "GettingStartedGuide", CreateAccount = true};
        PlayFabClientAPI.LoginWithCustomID(request, OnLoginSuccess, OnLoginFailure);
    }

    private void OnLoginSuccess(LoginResult result)
    {
         //>> Call Client API here <<
         var getStoreItemsRequest = new GetStoreItemsRequest { StoreId="[YourStoreId]" };// Please change this value to your own storeId from PlayFab Game Manager
        PlayFabClientAPI. GetStoreItems (getStoreItemsRequest, OnGetSuccess, OnGetFailure);

}
……
}