The trial that you installed is apparently from the creative cloud. If the develop module is disabled (i.e. you cannot use any of the tools) that is indicative that you have the creative cloud version. It will have to be removed because you cannot activate it with a serial number. It relies on an active creative cloud account, and is activated with your Adobe ID and password. There is no serial number associated with the creative cloud installation. Remove that version and install from here:
This is the standalone version, and it will prompt you for the serial number. Once you have installed it and entered your serial number, you can open up the catalog you have been using and all of the work that you have done will still be in place.
Many thanks Jim, this TOTALLY solved my problem! Spot on!
By following the Product updates link there is no LR 6.0 to download. Where did you find it.