I have Illustrator on my HP computer. I'm new to this app. I'm trying to use a calligraphy brush with the pressure option, but it's grayed out. The only ones I can select are fixed and random. I tried uninstalling and reinstalling Illustrator but that didn't work. I've been looking at what solved other people's problems and many of them have installed a Wacom driver, but I do not know what this is. Can someone please help?