question

The “Member” parameter in the JoinMatchmakingTicket request should be matching with one of the “MembersToMatchWith“ when calling CreateMatchmakingTicket API, otherwise it will return the error you mentioned in the title. You can use GetMatchmakingTicket API to check the “MembersToMatchWith“ attribute. Please check if your “Member” parameter (from line 10 to line 24 of your code snippet) is listed in the “MembersToMatchWith“ from the corresponding matchmaking ticket. If you still encounter this issue after that, you can provide your title id, the issue matchmaking ticket id, and your request parameters of the relevant API calls for us to troubleshoot.

look this is my Client have two player first player create matchmaking and second player joining.

 private static HttpClient client = new HttpClient();
static async Task Main(string[] args)
        {
            PlayFabApiSettings instanceSetting = new PlayFabApiSettings { TitleId = "7DCFA" };
            var clientAPIInstance = new PlayFabClientInstanceAPI(instanceSetting);
            await LoginIn(clientAPIInstance);
        }
        public static async Task LoginIn(PlayFabClientInstanceAPI instance)
        {
            PlayFabResult<LoginResult> result = await instance.LoginWithPlayFabAsync(new LoginWithPlayFabRequest
            {
                Username = "player1",
                Password = "*****"
            });
            string id = result.Result.EntityToken.Entity.Id;
            string type = result.Result.EntityToken.Entity.Type;
            var request = new HttpRequestMessage(HttpMethod.Get, $"http://localhost:64310/playfab/CreateNewMatchmaking?entityId={id}&entityType={type}");
            HttpResponseMessage response = await client.SendAsync(request);
            string ticketId = await response.Content.ReadAsStringAsync();
            PlayFabResult<LoginResult> result2 = await instance.LoginWithPlayFabAsync(new LoginWithPlayFabRequest
            {
                Username = "player2",
                Password = "****"
            });
            string id2 = result2.Result.EntityToken.Entity.Id;
           
            var request2 = new HttpRequestMessage(HttpMethod.Get, $"http://localhost:64310/playfab/JoinPlayer?entityId={id2}&entityType={type}&ticketId={ticketId}");
            HttpResponseMessage response2 = await client.SendAsync(request2);
            string ticketId2 = await response2.Content.ReadAsStringAsync();
        }

and this server side my web api which connect at PlayFab API

Create Matchmaking

 public async Task<PlayFabResult<CreateMatchmakingTicketResult>> Create(string entityId, string entityType)
        {
            return await PlayFabMultiplayerAPI.CreateMatchmakingTicketAsync(new CreateMatchmakingTicketRequest
            {
                Creator = new MatchmakingPlayer
                {
                    Entity = new EntityKey
                    {
                        Id = entityId, //7A83B7F4C80410AA
                        Type = entityType, //title_palyer_account
                    },


                    Attributes = new MatchmakingPlayerAttributes
                    {
                        DataObject = new
                        {
                            Skill = 24
                        }
                    },
                },
                MembersToMatchWith = new List<EntityKey>
                {
                    new EntityKey { Id = "1A6090C723BCC7D4", Type = entityType },                                           
                },
                GiveUpAfterSeconds = 120,
                QueueName = "PVP",
            });
        }

And Join

 public async Task<JoinMatchmakingTicketResult> JoinOtherPlayers(string entityId, string entityType, string ticketId)
        {
            PlayFabResult<JoinMatchmakingTicketResult> result = await PlayFabMultiplayerAPI.JoinMatchmakingTicketAsync(
                 new JoinMatchmakingTicketRequest
                 {
                     TicketId = ticketId,
                     QueueName = "PVP",
                     Member = new MatchmakingPlayer
                     {
                         Entity = new EntityKey
                         {
                             Id = entityId, //1A6090C723BCC7D4
                             Type = entityType //title_player_account
                         },
                         Attributes = new MatchmakingPlayerAttributes
                         {
                             DataObject = new
                             {
                                 Skill = 24
                             }
                         }
                     }
                 });
            return result.Result;
        }