How to use Dapper to execute a Postgre function?

I get an error when I try to call a postgre function using dapper. Where did i do wrong? I'll be glad if you can help me.

Error Message:

 availability_list(facilityId => integer, startDate => timestamp without time zone, endDate => timestamp without time zone) does not exist"

Call postgre function using Dapper:

var func = "public.availability_list";

var result = db.Query<ReportResponse>(
            sql: func,
            param: new { facilityId = request.FacilityId, startDate = 
            DateTime.Now, endDate = DateTime.Now },
            commandType: CommandType.StoredProcedure, 
            commandTimeout: 900) as List<ReportResponse>;

My Postgre Function:

CREATE FUNCTION Availability_List(facilityId int, startDate date, endDate date)
RETURNS report_type[] 
AS 
$$

DECLARE
result_record report_type[];

BEGIN

result_record := array(  
SELECT...
);

RETURN result_record;

 END $$ LANGUAGE plpgsql;