var ts = Model.Content.Site().FirstChild("trainingSchedule").Children("trainingScheduleItem")
.Where(x => x.IsVisible())
.Where(x => x.GetPropertyValue("programKey").ToString() == pk)
.GroupBy(x => x.GetPropertyValue("grouping"))
  .OrderBy(x => x.GetPropertyValue<DateTime>("startDate")); // This line is breaking the page - why?

This was created using the query builder tool - I added an additional .Where clause and the GroupBy and everything works great, until I add the .OrderBy. Now I'm getting the message:

CS1928: 'System.Linq.IGrouping<object,Umbraco.Core.Models.IPublishedContent>' does not contain a definition for 'GetPropertyValue' and the best extension method...

Can anyone tell me what I'm missing?

UPDATE: Moved the OrderBy clause to above the GroupBy... new and working code below. Took me 6+ hours to figure that out - maybe this will help someone else.

var ts = Model.Content.Site().FirstChild("trainingSchedule").Children("trainingScheduleItem")
    .Where(x => x.IsVisible())
    .Where(x => x.GetPropertyValue("programKey").ToString() == pk)
    .OrderBy(x => x.GetPropertyValue<DateTime>("startDate")
    .GroupBy(x => x.GetPropertyValue("grouping"))
      );