Licensing a trial of PSE 15.0 with a product key

In July 2017, I downloaded a trial of PSE 15.0. On August 5th, I purchased a product key. I have a Creative Cloud subscription. I have the PSE 15.0 serial number and nowhere to input it.

Twice since then PSE Elements 15.0 has stopped working, giving me the error message "Elements Organizer is not currently licensed. Please launch Photoshop Elements Editor to update the licensing information." I can launch the editor, but THERE IS NOWHERE IN THE EDITOR TO INPUT THIS INFORMATION! I am signed into the application with my Adobe User Name and Password. I have signed out/signed back in/ rebooted, etc. Nothing works. The organizer fails to open. I HAVE A SERIAL NUMBER and nowhere to input it.

Twice since then, I have contacted Adobe online help to convert this trial using my serial number, and twice Adobe online help has "entered" my computer to try to fix the problem. Both times, after HOURS and HOURS on hold and online, the techs were able to fix the problem, and now TWICE since then, another few weeks go by , and I get the SAME PROBLEM with the SAME ERROR MESSAGE.  I really need this issue fixed once and for all. I spend hundreds of dollars on Adobe products and subscriptions every year and have valid serial numbers. This has to stop. Please help. Thank you.