you have the purchased (not subscription) version of lr 6 (as an upgrade to lr5).
if all that is correct, then when you start your lr 6 installation you will be prompted for the lr 6 serial number. if lr 5 is not installed you will then be prompted for your lr 5 serial number.
if you enter the correct number at the correct time, installation should proceed.
I succeeded!
But as LR5 was also an upgrade, I had to find and enter my first activation number of my first Lightroom, the LR4, before the LR6 one was accepted...!
Crazy but true...
But happy now; but I've lost one hour or more...
Bye!