I complete Main Quest: Imperial Audience and then go through all the exercises prior to departing the castle. When I approach the caslte exit...double doors....I get the hot key symbol/fast travel. Cannot fast travel from here so am dead in the water....have done a reinstall and everything I can think of to no avail. Anyone else have this problem and what did you do? Thanks!

Not entirely sure what you mean jim. Are you on PC? Keyboard & Mouse or controller?
The key for fast travel is E with KB & M on PC. It's been quite a while since I did that part, so I don't remember the specifics of exiting the castle. If it prompts E you just use it to open the doors.
I think we need more specific information to have a reasonable chance of offering advice. :)

Don't know if it's the right place(where you are stuck).
Now that you're properly prepared for the adventures ahead, examine the large double doors to bring up the fast travel map. Switch to the world map view, then zoom in on Velen - No Man's Land. You've only one fast travel option here at the moment, so select the Hanged Man's Tree to begin your search for Hendrik, the emperor's spy.

Thanks for the replies....I finally figured out that exact placemnt of my cursor while trying to fast travel was required...the signpost that showed up on the castle map worked. This probably will not be my last issues....thanks again!

Your kidding,now that's unbelieveable:-)