Execute sql task

insert into [DEV_BI].dbo.[DimAktivitet]([Beskrivning],[företag],[Projektnummer],[Aktivitet],loaddate)  
  select null,a.DATAAREAID,a.PROJID,a.MID_ACTIVITYNUMBER,GETDATE() from 
  [?].dbo.[v_ProjCostTrans_ProjEmplTrans] a 
  left outer join 
  [DEV_BI] .dbo.[DimAktivitet] b
  on a.MID_ACTIVITYNUMBER = b.Aktivitet AND a.DataAreaID = b.företag AND a.ProjID = b.Projektnummer
  where b.Aktivitet_key is null

I have this above sql code in execute sql task and in the parameter mapping i have mapped a variable named user::connectionstring with data type nvarchar , parameter name = 0.  Im getting this following error.

[Execute SQL Task] Error: Executing the query "insert into [DEV_BI].dbo.[DimAktivitet]([Beskrivni..." failed with the following error: "Invalid object name '?.dbo.v_ProjCostTrans_ProjEmplTrans'.". Possible failure reasons: Problems with the query, "ResultSet" property not set correctly, parameters not set correctly, or connection not established correctly.

please someone help me to solve this.