could anyone give me a help in this exception ?

The fieldName you've passed in doesn't represent a property with one of the supported types:

• int / Nullable<int>
• long / Nullable<long>
• float / Nullable<float>
• double / Nullable<double>
• decimal / Nullable<decimal>

Those are the only types which can be processed by the Sum method. For any other property type, you'll need to convert the property value to a suitable type.

Something like this should work:

var parameter = Expression.Parameter(query.ElementType, string.Empty);
var propertyInfo = query.ElementType.GetProperty(fieldName);
var propertyAccess = Expression.MakeMemberAccess(parameter, propertyInfo);

Expression body;
Type propertyType = propertyAccess.Type;
Type baseType = Nullable.GetUnderlyingType(propertyType); // Returns null if the type is not Nullable<T>

switch (Type.GetTypeCode(baseType ?? propertyType))
{
    case TypeCode.Int32:
    case TypeCode.Int64:
    case TypeCode.Single:
    case TypeCode.Double:
    case TypeCode.Decimal:
    {
        // Supported type - use the property directly:
        body = propertyAccess;
        break;
    }
    case TypeCode.Byte:
    case TypeCode.SByte:
    case TypeCode.Int16:
    case TypeCode.UInt16:
    {
        // Small enough to fit in an Int32:
        Type targetType = typeof(int);
        if (baseType != null) targetType = typeof(Nullable<>).MakeGenericType(targetType);
        body = Expression.Convert(propertyAccess, targetType);
        break;
    }
    case TypeCode.UInt32:
    {
        // Convert to an Int64:
        Type targetType = typeof(long);
        if (baseType != null) targetType = typeof(Nullable<>).MakeGenericType(targetType);
        body = Expression.Convert(propertyAccess, targetType);
        break;
    }
    default:
    {
        // Unknown type - try converting to Double.
        // If there is no conversion, this will throw an exception.
        
        Type targetType = typeof(double);
        if (baseType != null) targetType = typeof(Nullable<>).MakeGenericType(targetType);
        body = Expression.Convert(propertyAccess, targetType);
        break;
    }
}

var propertyAccessExpression = Expression.Lambda(body, parameter);

var expression = Expression.Call(
    typeof(Queryable),
    "Sum",
    new Type[] { query.ElementType },
    query.Expression,
    Expression.Quote(propertyAccessExpression)
);

return query.Provider.Execute(expression);

Also, there is no built-in Count extension method for the non-generic IQueryable interface. Unless you've created your own, or imported a third-party version, the first line of your method won't work.