How to use average function with EF

Also as per your query the Include method is used to load related entities along with the main entity, but in your query, you are trying to include the result of the Average operation which is not a related entity. Instead of using Include, you can use Select to project the result of the query into the desired shape.

Another approach : 

var result = await _context.transactions
    .Where(b => b.SId == request.Id)
    .GroupBy(a => a.RawDateTime.Date)
    .Select(g => new
    {
        Date = g.Key,
        AverageScore = g.Average(v => v.Score)
    })
    .ToListAsync();

    
    
This query groups the transactions by date, calculates the average score for each group.

var transactionsByDate = await _context.transactions
    .Where(t => t.SId == request.Id)
    .GroupBy(t => t.RawDateTime.Date);

var averageScoresByDate = transactionsByDate
    .Select(group => new
    {
        Date = group.Key,
        AverageScore = group.Average(t => t.Score)
    });

var result = await averageScoresByDate.ToListAsync();

await _context.transactions
    .Where(b => b.SId == request.Id)
    .GroupBy(a => a.RawDateTime.Date)
    .Select(g => new {
        Date = g.Key,
        AverageScore = g.Average(a => a.Score)
    })
    .ToListAsync();

Please try below,

            var resultGroupBy = await _context.transactions
                                .Where(x => x.SId == request.Id)
                                .GroupBy(g => g.RawDateTime.Date)
                                .Select(grp => new
                                {
                                     Date = grp.Key.Date,
                                     AvgScore = grp.Average(v => v.Score)
                                }).ToListAsync();

            foreach (var item in resultGroupBy)
            {
                Console.WriteLine("Date :{0}", item.Date);
                Console.WriteLine("AVG Score :{0}", item.AvgScore);
            }

You're using the Include method after the GroupBy method, which is not allowed in Entity Framework. The Include method is used to specify related entities to be included in the query results, but it should be used before any grouping or aggregation operations.

var result = await _context.transactions
                .Where(b => b.SId == request.Id)
                .GroupBy(a => a.RawDateTime.Date)
                .Select(g => new 
                {
                    Date = g.Key,
                    AverageScore = g.Average(v => v.Score)
                })
                .ToListAsync();