Dynamic ORDER BY And UNION

Hello,

I am trying to use dynamic ORDER BY with UNION in a stored procedure but I keep getting this error message:

ORDER BY items must appear in the select list if the statement contains a UNION, INTERSECT or EXCEPT operator.

The ORDER BY works fine without the UNION, and the UNION works fine without the dynamic ORDER BY (just putting an ORDER BY works fine)

Here is the whole query in case this helps, thank you

SELECT
leave_id, leave_type, annualLeave.from_date,
convert(nvarchar(10), annualLeave.request_date, 103) as dated,
convert(nvarchar(10), annualLeave.from_date, 103) as from_date,
convert(nvarchar(10), annualLeave.to_date, 103) as to_date,
annualLeave.canceled,
annualLeave.working_days,
staff.staff_name,
Case WHEN annualLeave.canceled = 'True' THEN 'canceledPO'
WHEN annualLeave.authorized_mng is not NUll AND annualLeave.authorized_hr is not Null THEN 'greenItem'
ELSE ''
END AS pendingStyle,
Case WHEN annualLeave.canceled = 'True' THEN 'canceledPO'
ELSE ''
END AS historyStyle
FROM annualLeave
INNER JOIN staff ON annualLeave.staff = staff.staff_id
WHERE staff.department like @Department
and (@staffID is Null or annualLeave.staff = @StaffID)
and (@Canceled is Null or annualLeave.canceled = @Canceled)
and (@AuthorizedMng is Null or annualLeave.authorized_mng is Null)
and (@AuthorizedHR is Null or annualLeave.authorized_hr is Null)
and (@StartingDate is Null or annualLeave.from_date > @Startingdate)
and (@IsPastLeave is Null or annualLeave.from_date < @IsPastLeave)
and (@EndingDate is Null or annualLeave.to_date <= @EndingDate)

UNION

SELECT
leave_id, leave_type, annualLeave.from_date,
convert(nvarchar(10), annualLeave.request_date, 103) as dated,
convert(nvarchar(10), annualLeave.from_date, 103) as from_date,
convert(nvarchar(10), annualLeave.to_date, 103) as to_date,
annualLeave.canceled,
annualLeave.working_days,
staff.staff_name,
Case WHEN annualLeave.canceled = 'True' THEN 'canceledPO'
WHEN annualLeave.authorized_mng is not NUll AND annualLeave.authorized_hr is not Null THEN 'greenItem'
ELSE ''
END AS pendingStyle,
Case WHEN annualLeave.canceled = 'True' THEN 'canceledPO'
ELSE ''
END AS historyStyle
FROM annualLeave
INNER JOIN staff ON annualLeave.staff = staff.staff_id
WHERE staff.department like @Department
and (@staffID is Null or annualLeave.staff = @StaffID)
and (@Canceled is Null or annualLeave.canceled = @Canceled)
and (@AuthorizedMng is Null or annualLeave.authorized_mng is Null)
and (@AuthorizedHR is Null or annualLeave.authorized_hr is Null)
and annualLeave.leave_type like 'Leave in lieu'
ORDER BY
CASE @OrderDir
WHEN 'ASC' THENannualLeave.from_date
END
ASC,
CASE @OrderDir
WHEN 'DESC' THEN annualLeave.from_date
END
DESC