Reporting Services :: Pass Selected Parameter To Store Procedure Used To Populate Filter

I am working on a report where some customization is need to be delivered.. situation is , i have some parameter in report @USER_ID , @Report_Type where i am proving selection to user to select Report Type (Pending Or Completed) and passing USER_ID auto matically from URL string of user login(C# code).

I have another parameter @USER_IDS which is multiple selection for user and it will be filled with the users which lie under passed @USER_ID means i just need to add dataset with the query to select users from mapper table where reporting_head =@USER_ID, simple, but i have requirement to populate the underlined users with the selection of @USER_ID and @Report_Type and it need some TSQL code to populate so i am using Another store procedure and using same parameter as my main store procedure has .

Now i am using dataset with this store procedure  to fill my @USER_IDS  parameter 

Both parameters value will be passed from main report parameters now , when i am previewing a report i am getting error

and i also tried to write exec statement in dataset query with the main repport paramters but exec is not supported ..