If you had LR installed before when it was the serial-number version, then uninstall it, and install LR from the CC Desktop app.
If you have never had LR installed, before, then there may be something wrong with your subscription being recognized.
If you log into www.adobe.com <http://www.adobe.com> and click Manage Account what does it show for your subscriptions?
Thank you for your reply. Yes, that's exactly the problem. My subscription is not being recognized. My subscriptions area of MyAccount doesn't show anything. However, Photoshop is working fine and the monthly subscription fee is charged just in time every month. Nobody in Adobe support seems to know what to do. Adobe Care suggested deleting the ppm.db file in the OOBE folder, but that file is non-existent. What can I do?