merging response from 1st call using flatMap into 2nd call gives error [unsupported media type status 415] --

Question

In my app, I am making a http.post request and getting back a value of 'gradeS'. Now I wanted to use this value of gradeS as parameter to make another cascaded call and fetch some list of names. I'll post the code I have written so far --

 1. my first call [originally]

            this.http.post('api/SampleData/CustomerGradeByID', JSON.stringify(body), { headers: _Headers })
            .subscribe((result: any) =>
            {
                this.gradeS = result.json() as any[]
            },
            (error: any)=>console.error(error))


 2. my service function

    export class CommListService {
    endPoint: string;

    constructor(private http: Http) {
        this.endPoint = "/api/SampleData/MakeCommunicationList";
    }

    GetfacilityNames(grade: string): Observable<Communication[]> {
        var body = grade;

        var _Headers = new Headers();
        _Headers.append('Content-Type', 'application/json');

        var commList = this.http.post(this.endPoint, JSON.stringify(body))
            .map((r: Response) => {
                return (r.json().length != 0 ? r.json() : [{"facility_name": "No Record Found" }]) as Communication[]
            });
        return commList;
    }
}

 3. my model class

    export interface ICommunication {
        facility_name: string;
    }

    export class Communication implements ICommunication {
        facility_name: string;
    }

 4. how I tried to merge

 this.http.post('api/SampleData/CustomerGradeByID', JSON.stringify(body), { headers: _Headers })
        .do(u => this.gradeS = u)
        .flatMap(u => this.commu.GetfacilityNames(this.gradeS))
        .subscribe(p => this.comms = p);


 5. finally in my html 

     <ul>
         <li *ngFor="let cm of comms">{{cm.facility_name}}</li>
     </ul>

SO where did I go wrong? I am getting data from database. By running a stored procedure. Whats' with the 415 status? What does it mean in this context? Could there be something wrong in the server side C# code? I have a hunch that I could have written something wrong at the server side stuff the backend comprising of C# and MSSQL. Although I am not sure.

this is my C# code

public JsonResult MakeCommunicationList([FromBody]string grade)
        {
            var gId = _appDbContext.CustomerGrades.First(m => m.grade_name == grade);
            try
            {
                var inpParam = new SqlParameter("gId", SqlDbType.BigInt) { Direction = ParameterDirection.Input };

                var salesData = (_appDbContext.Database.ExecuteSqlCommand($"EXEC {"sp_GetCommunicationList"} @gId", inpParam));
                return Json(salesData);
            }
            catch (Exception exp)
            {
                return Json(exp.GetBaseException());
            }
        }

and this is my SP

CREATE procedure [dbo].[sp_GetCommunicationList]

@gId bigint

as
begin

begin try

    if exists (
            Select * from tempdb..sysobjects Where id = object_id('tempdb.dbo.#temp')
          )
        DROP TABLE #temp

else
        select facility_name
        into #temp 
        from Facilities ff
        join
        GradeFacilities gf
        on
        gf.facility_id=ff.Id
        where
        grade_id=@gId

--      select * into #temp from Orders where customer_id=@customer_id
end try

begin catch
    select error_message(),
           error_severity(),
           error_number(),
           error_state()
end catch

end

Please Help!! I am unable to figure this out!