question

Firstly, you need to reference the two namespaces mentioned above. Secondly, the API of PlayFab is called asynchronously in c#, and UpdateUserInternalData has no response. I modified your code, you can refer to it.

It is worth mentioning that the commented part is the configuration of the environment. If you have already configured it, please ignore it. If you don't know how to configure it, you can refer to Where to set DeveloperSecretKey for Azure PlayFabServerAPI calls - Playfab Community.

using System.Net.Security;
using System.Diagnostics;
using System;
using System.IO;
using System.Collections.Generic;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json;
using PlayFab.ServerModels;
using PlayFab.Json;
using PlayFab.DataModels;
using PlayFab.Internal;

using PlayFab.Samples;
using PlayFab;

namespace Company.Function
{
    public static class UpdateSession
    {
        [FunctionName("UpdateSession")]
        public static async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Anonymous, "get", "post", Route = null)] HttpRequest req,
            ILogger log)
        {
            FunctionExecutionContext<dynamic> context = JsonConvert.DeserializeObject<FunctionExecutionContext<dynamic>>(await req.ReadAsStringAsync());
            var request = new UpdateUserInternalDataRequest
            {
                PlayFabId = context.CallerEntityProfile.Lineage.MasterPlayerAccountId,
                Data = new Dictionary<string, string>()
                {
                    {"abc", "abc"}
                }
            };
            //PlayFabSettings.staticSettings.TitleId = context.TitleAuthenticationContext.Id;
            //PlayFabSettings.staticSettings.DeveloperSecretKey = Environment.GetEnvironmentVariable("PLAYFAB_DEV_SECRET_KEY", EnvironmentVariableTarget.Process);
            await PlayFabServerAPI.UpdateUserInternalDataAsync(request);
            return null;
        }
    }
}