How to used GroupBy and select column in async

Question

ERD

Sorry I'm just learning to use C#, How do I create a grouping function using async ? I want to add up the contract values, I tried with code like this

public async Task <IEnumerable<BudgetingData>> GetTotalBudgeting()
        {
            var result = await _context.Budgetings
                .Include(e => e.ProposalBudgeting)
                .Include(e => e.ProposalBudgeting.Division)
                .GroupBy(e => e.ContractValue)
                .AsNoTracking()
                .ToListAsync();
            return result;
        }

Budgeting Data

public class BudgetingData
{
    [Key]
    public int Id { get; set; }
    public int? ProposalBudgetingId { get; set; }
    public string SubBudgetingProject { get; set; }
    public string ValueProjectEstimation { get; set; }
    public string CurrentYearAllocation { get; set; }
    public string Procurement { get; set; }
    public string StatusProject { get; set; }
    public string ContractValue { get; set; }
    public string LstYear_Realization { get; set; }
    public string LstYear_RemainingPayment { get; set; }
    public string ThsYear_TotalPayment { get; set; }
    public string ThsYear_RemainingPayment { get; set; }
    public DateTime LastUpdate_dt { get; set; }
    public int? StatusId { get; set; }
    public string IsDelete { get; set; }

    public virtual Status Status { get; set; }
    public virtual ProposalBudgeting ProposalBudgeting { get; set; }
}

I want to total the contract value of the divisions, for example division 1 has a total contract value of 1000

However, when the error returns result, the error is "Cannot implicitly convert type 'type' 1 'type 2'. An explicit conversion exists (are you missing a cast?)"