Quoted from Victoria Bampton aka The Lightroom Queen-
- Lightroom now requires online activation.
You can have 2 machines activated at a time. If you try to log in on a 3rd machine, it asks if you want to deactivate the other machines remotely, so you don’t have to remember to deactivate or go through a nightmare phone procedure to unlock it.
So continue with activation process of the 'new' Macbook and re-activate the other Mac,
When you will install that in your new machine, it will give you the option to sign out from other machines.
So your license will stay with 2 machines.
Let us know if you still not able to do that.
You are Welcome