The DigiLink I/O license is supposed to come with the 12.6 upgrade (download) In my case, it did not... my legacy "Blue" 192 will not work with PT 12.6 without it. Been trying to get support from Avid but there has been no response as of yet.
Also, of note and caution to others, I had the 192 connected when I started up PT 12.6... didn't know the DigiLink license was not part of the PT 12.6 download package... upon install and launch of PT 12.6 I got an AAE -1125 error and then within the next 30 seconds came a screaming horrible static sound out of the 192 and every LED on the device was brightly pegged to the last LED...
thought I toasted it but it's Ok running with my old rig. Don't know if I toasted the HDX card yet... aargh I got the Upgrade Blues™.