Your statement does not make sense.
UNION will remove duplicates from the total result, as you select the identical result two times, the result will be exactly the same as without the union - so it will simply return all rows from CMTRAVELINSTRUCTION. If you want to keep duplicates you need to use UNION ALL.
Having said this, your error is caused by the fact that an ORDER BY can only be applied to the complete result of a UNION, not to single parts of it.
And you probably need to reference the sort column using the column index not its name (not sure about that though), e.g:
SELECT * FROM first_table
SELECT * FROM second_table
ORDER BY 1 DESC;
Just as an aside, for an adhoc query something like "select *" is fine, but never use it for any production code. If you are going to pursue a career in oracle, get into the habit of specifying the columns.
You do not need a parachute to skydive. You only need a parachute to skydive twice.