Try this
DECLARE @DateDiff AS Int
SELECT @DateDiff = DATEDIFF(Day,@StartDate,@EndDate)
INSERT INTO Holidays(EmployeeID, StartDate, EndDate, Duration, PE) --< columns name for PE
VALUES(@EmployeeID, @StartDate, @EndDate, @DateDiff,'PE')
OR
INSERT INTO Holidays(EmployeeID, StartDate, EndDate, Duration, PE) --< columns name for PE
VALUES(@EmployeeID, @StartDate, @EndDate, DATEDIFF(Day,@StartDate,@EndDate),'PE')