question

The EntityKey in the AddInventoryItems API request should be title_player_account not the master_player_account. And since you use title entity token for authentication, player permission in the Economy Settings page won’t have an effect. Also, here is the code example you can refer to:

 public static class TestFunction1
     {
         [FunctionName("Itemtest")]
          public static async Task Run(
              [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
              ILogger log)
          {
              FunctionExecutionContext<dynamic> context = JsonConvert.DeserializeObject<FunctionExecutionContext<dynamic>>(await req.ReadAsStringAsync());
        
              var apiSettings = new PlayFabApiSettings
              {
                  TitleId = Environment.GetEnvironmentVariable("PLAYFAB_TITLE_ID", EnvironmentVariableTarget.Process),
                  DeveloperSecretKey = Environment.GetEnvironmentVariable("PLAYFAB_DEV_SECRET_KEY", EnvironmentVariableTarget.Process)
              };
                  
                  
              var titleAuthContext = new PlayFabAuthenticationContext();
              //set the authentication Token to Title Token and no need to set the EntityKey
              titleAuthContext.EntityToken = context.TitleAuthenticationContext.EntityToken;
        
              PlayFabEconomyInstanceAPI api = new PlayFabEconomyInstanceAPI(apiSettings,titleAuthContext);
        
              var result = await api.AddInventoryItemsAsync(new PlayFab.EconomyModels.AddInventoryItemsRequest
              {
                  Item = new PlayFab.EconomyModels.InventoryItemReference
                  {                    
                      AlternateId=new PlayFab.EconomyModels.AlternateId
                      {
                          Type= "FriendlyId",
                          Value= "the FriendlyName of the item"
                      } 
                  },
                  Amount = 3,
                  //set the entitykey to the title_player_account of the user you want to modify the inventory for.
                  Entity = new PlayFab.EconomyModels.EntityKey{
                      Id = context.CallerEntityProfile.Entity.Id,
                      Type=context.CallerEntityProfile.Entity.Type
                  }
              }); ;
          }
     }