I purchased lightroom 6 and have the serial number but can't figure out how to put it on my new computer. Please help me .
(from another user with Creative Cloud- My reply assumes you are a 'new' user to Lightroom)
How did you purchase? On a CD or by a download?
In either case, you probably should be connected to the internet and download the latest version 6.7,
then 'open/run' the setup file.
After the 'setup' installation has completed you should open/run the Lightroom program when it will ask you for the serial number and register your program online for perpetual use. Lightroom will ask you to create a new empty catalog, after which you can start importing your image data, and have a lot to learn.