Hi Shivers,
What is the version of Captivate you are using (Go to Help > About Adobe Captivate) ?
Have you tried to Right click on Captivate icon and then click on Run as Admin and then publish?
Have you tried with any test project?
Are you uploading it directly on youtube?
Where exactly you are saving your published mp4 file?
Thanks and Regards
Himanshu Satija