I had this same problem (PGP Desktop on a Mac) when PGP was owned by PGP Corp.
The issue was that when the software was installed it would "phone home" to a license authorization
server and checked it that license was already in use. There was a process I had to go through to remove the software from Computer A first then install on Computer B. I also recall it didn't work but the tech support folks were able to reset the license server.
That said, I'm wondering if your PGP software is doing something similar so that it knows that the license is already in use on Computer A, so it won't let you use that on computer B.
Have you tried logging into the Symantec licensing server and check the status of your license?
<https://licensing.symantec.com/>
That might provide some information, but if failing that...
as Tom suggests you should call tech support.