The Help Manual tells me the following, but there is no place to contact adobe.
Please unregister all my old computer.
Have you lost access to your previous computer?
If you cannot access the previous computer on which you installed the app—whether it was stolen, was lost, crashed, or locked—you need to contact Adobe to deactivate the app.
If you upgraded your computer’s hardware, replaced a hard drive, upgraded your operating system, or reformatted your computer before deactivating your software, you may have disrupted the activation connection.
That's because Lightroom 6 is discontinued and unsupported now.
If you need an activation count reset, contact Adobe by online chat and ask for one. DO NOT ASK FOR ANY OTHER ASSISTANCE. Owing to the Christmas holiday break, wait times may be longer than usual until after the new year.
CONTACT ADOBE SUPPORT:
Online Chat: https://helpx.adobe.com/contact.html?rghtup=autoOpen
Phone M-F, during normal business hours: https://helpx.adobe.com/contact/phone.html
Important: posting in the forum does NOT contact Adobe.
Any private reply saying it is from Adobe will be a fake Adobe.