I recently bought a Huion Kanvas Pro 12. It works marvelously with Photoshop, but in Adobe Illustrator, the calligraphic brushes options for the pressure are greyed out. Any brush I use, even the one with Pressure or Angle already configured don't work. I'm using Adobe Illustrator CC 25.1 and my Huion Tablet Driver are up to date. I also tried to reset the preferences of Illustrator, but it didn't work.
I tried to install the Wacom Driver. Sadly, without success either. I already have another graphic tablet and it works fine. The other one is a Wacom, though. Before you ask, I did unplug the Wacom one before testing. I must say, it's getting tiresome.
Contact Huion and have them guide you through the installation and setup process. Yes, that is tiresome and yes, it's necessary. It's not straightforward to install any graphic tablet driver. I've been through it with them just a couple months ago (the tablet works now). Have them guide you. They do that.