Recently my LR (box version) started giving me problems on MacBook so I decided to install in on my new Windows on another device. I logged out and deleted the app from Mac, but Win version communiates that the serial number is already in use and I can't use LR on my new laptop. I've checked my adobe account and I can see my product with serial number, but no device is registered.
Please advise the exact version number of Lightroom you are trying to install. Also the version number of your operating system.
In the event you are referring to Lightroom 6, be aware that this version is not fully supported by Adobe and users no longer have the option of deactivating / activating an installation and it's necessary to contact Adobe direct via chat or telephone.
If you click on this link you can get assistance from an Adobe staff member who can assist, @Carissa Massongill .
Send her a private message with your Adobe ID info.