What computer operating system is your Premiere Elements 12 running on? Have you updated 12 to the 12.1 Update using an opened projects Help Menu/Update. If not, please do so.
Let us go through the usual drill...
a. Latest version of QuickTime installed on your computer with Premiere Elements 12/12.1?
b. Running program from User Account with Administrative Privileges?
c. Is everything else working in Premiere Elements except for its YouTube feature?
d. Does the problem exist with and without your antivirus and firewall(s) disabled?
Then the drill specific to Publish+Share/Social Websites/YouTube...
a. What is the estimated file size and duration of the Timeline that you are trying to upload to YouTube from the Premiere Elements feature?
What type of formats are present on that Timeline?
b. Please check out your YouTube account..... by exporting your Premiere Elements 12/12.1 Timeline to a .mp4 file saved to the computer hard drive.
Then upload that .mp4 to YouTube at the YouTube web site. Do you have a regular account at YouTube or one of its time extended ones?
c. Can you describe any differences between the 1 video upload that worked from within Premiere Elements feature versus the 1 that did not.
If you start a new project for the one that did not work, does it then work?
Please review and consider. Your answers will help us evaluate the situation for us to come up with a solution for you.