question

I have tested the RemoveMembers API with title entity token, it works fine. And the error message indicates that your AuthenticationContext may be set incorrectly. Also, here is the working code you may refer to:

  public static class RemoveGroupMember
         {
             [FunctionName("RemoveGroupMember")]
             public static async Task<dynamic> Run(
                 [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
                 ILogger log)
             {
                 var context=JsonConvert.DeserializeObject<FunctionExecutionContext<dynamic>>(await req.ReadAsStringAsync());    
                    
        
                 PlayFabSettings.staticSettings.TitleId=Environment.GetEnvironmentVariable("PLAYFAB_TITLE_ID",EnvironmentVariableTarget.Process);
                 PlayFabSettings.staticSettings.DeveloperSecretKey=Environment.GetEnvironmentVariable("PLAYFAB_DEV_SECRET_KEY",EnvironmentVariableTarget.Process);
        
                 var titleContext=new PlayFabAuthenticationContext()
                 {
                     EntityToken=context.TitleAuthenticationContext.EntityToken,
                     EntityId=Environment.GetEnvironmentVariable("PLAYFAB_TITLE_ID",EnvironmentVariableTarget.Process),
                     EntityType="title"
                 };
        
                 var result = await PlayFabGroupsAPI.RemoveMembersAsync(new PlayFab.GroupsModels.RemoveMembersRequest{
                     AuthenticationContext=titleContext,
                     Group=new PlayFab.GroupsModels.EntityKey{
                         Id="xxxx"
                     },
                     Members=new List<PlayFab.GroupsModels.EntityKey>{
                         {
                             new PlayFab.GroupsModels.EntityKey{
                                 Id="xxxx",
                                 Type="title_player_account",                          
                             }
                         }
                     }
                 });
                    
                 return result;
             }
         }