API - problemas com await e async

// POST: api/Itemprevendas
        [ResponseType(typeof(Itemprevenda))]
        public async Task<IHttpActionResult> PostItemprevenda(Itemprevenda itemprevenda)
        {

            if (!ModelState.IsValid)
            {
                return BadRequest(ModelState);
            }

            //  Implementar o id.
            decimal cd = await GetMaiorCod();
            itemprevenda.Cditemprevenda = cd;

            db.Itemprevenda.Add(itemprevenda);

            try
            {
                await db.SaveChangesAsync();
            }
            catch (DbUpdateException)
            {
                if (ItemprevendaExists(itemprevenda.Cditemprevenda))
                {
                    return Conflict();
                }
                else
                {
                    throw;
                }
            }

public async Task<decimal> GetMaiorCod()
        {
            Itemprevenda itemprevenda = await db.Itemprevenda.FindAsync(1);
            if (itemprevenda == null)
            {
                return 1;
            }

            decimal GetTeste = 0;
            var itemQuery =
                from Itemprevenda in db.Itemprevenda
                group Itemprevenda by Itemprevenda.Cditemprevenda into grouping
                select new
                {
                    grouping.Key,
                    maiorCod =
                    from Itemprevenda in grouping
                    where Itemprevenda.Cditemprevenda == grouping.Max(Itemprevenda1 =>
                    Itemprevenda1.Cditemprevenda)
                    select Itemprevenda
                };
            foreach (var item in itemQuery)
            {
                foreach (var listing in item.maiorCod)
                {
                    GetTeste = (listing.Cditemprevenda + 1);
                }
            }
            return GetTeste;
        }