What version of Premiere Elements are you using and on what computer operating system is it running?
The generalization is this....
You cannot remove the Adobe watermark from an export of the tryout project (Timeline content export to a file saved to computer
hard drive or Timeline content burn to disc).
But you can remove the Adobe watermark from tryout projects (project.prel files). You must open your tryout project.prel files in the purchased Premiere Elements. Classically, next you go to Timeline Menu/Delete Rendered Files, and the Adobe watermark is gone. Some of us have found that just opening the tryout project.prel files in the purchased product automatically removes the Adobe watermark. See what works for you.
Please review and then let us know the outcome.