Just in case we need to get into some computer related details later, what computer operating system is your Premiere Elements 12 running on?
If you go to Expert workspace Help Menu/, do you find Sign In or Sign Out when you are having these Adobe watermark problems?
Given that you cannot remove the Adobe watermark from an export of the tryout Premiere Elements 12 tryout (either burn to disc or export to file saved to the computer hard drive). You can only remove the Adobe watermark from the tryout project file (project.prel).
The classical way to remove the Adobe watermark from the tryout project file is to open the tryout project file in the purchased Premiere Elements into which the purchased serial number has been inserted. And, then the user goes to Expert workspace/Timeline Menu/and clicks on Delete Rendered Files. In contrast, others including myself have found that just opening the tryout project file in the purchased product resulted in the automatic disappearance of the Adobe watermark. Did you try the both approaches to getting rid of the Adobe watermark in your tryout project(s)?
If you have been there and done all of the above, you could try some of the following paths...
a. I am assuming that you are running the program from a User Account with Administrative Privileges. If not already, apply Run As Administrator by right clicking the desktop icon for the program and selecting Run As Administrator.
b. Delete the Adobe Premiere Elements Prefs file and/or the whole 12.0 Folder in which it exists.
c. Deactivate the program within a project (Help Menu/Sign Out), uninstall (the usual Control Panel route), ccleaner run through, reinstall with antivirus and firewalls disabled.
More specific details will be forthcoming when you let us know the computer operating system involved.
Please review and consider.