Query Function, return value and not table...

Source = List.Dates(StartDate,1,#duration(1,0,0,0)),

Instead you could write {StartDate}. 

Is your weekend Saturday/Sunday? Better specify the FirstDayOfWeek parameter to avoid ambiguity in international setups.

Here's a simplified version that returns the scalar value you requested.

(StartDate)=>
  DateTimeZone.From(
    List.RemoveItems(
      List.Select(
        List.Dates(StartDate, 7, #duration(1, 0, 0, 0)), 
        each List.Contains({1 .. 5}, Date.DayOfWeek(_))
      ), 
      HolidayList
    ){0}
  )
    + #duration(0, 8, 0, 0)