Hi,
Are you trying to publish directly from Captivate. In this case there are chances that the publish will not work. This is a known issue and we are working on this. I insist you should opt following workflow to publish the content to youtube.
1) Publish content as mp4 to your system.
2) open youtube in browser and then upload it from there.
Why the direct publish is failing as there is some issue with the authentication part.
Please opt the workaround and let us know if issue still exist. if exist then please provide the steps you opted.
Thanks and Regards
Shriyansh
Adobe Captivate Team.