Possibly you are doing something wrong, but it is impossible to tell because of the lack of details. "I am unable..." is not an informative statement.

The Cathay web page asks for last name and firstname, or firstname and lastname, depending on which location-specific page you are using. This has caused me to enter info incorrectly once or twice.

Try checkmytrip.com to see if you can make it work there. The same PNR should work.

Of course, if you wrote the booking reference down incorrectly, no one can help you diagnose that. You have to call CX.