Hi Rokyo,
Welcome to our forums.
You may not have the non-free repository enabled on your system, so apt will not find the package you are looking for. Did you completed the step to add it to your /etc/apt/sources.list described in the article’s Required Repositories step?
This is realy cool! Asus Amd Radeon RX 480 works on Debian 9! Now I can play games like Lef4 Dead, Half Life.
But, I have trouble: It is no HDMI sound 
AMD HDMI sound has priority in Debian’s Audio devices and has 100% level of volume, but although i do not hear sound
Are any solves ?
Thank you!
ThankyouThankyouThankyou!!
Just spent five minutes following your instructions, word for word, and ended a 6 hour quest/obsession to get my Craigslist Special integrated graphics (Gigabyte GA-F2A85X-UP4, A10-5800K APU w/ Radeon HD7660D graphics) to set the resolution correctly! It actually worked the first time! That’s never happened in the history of the internet!
You are a GOD! If you print a meatloaf recipe, that’s all I’ll eat for the rest of my life! It will be THAT good!
Cheers!
As of June 2019, if you do the full 3-DVD install of Debian 9.9.0, there is an unofficial Disc 1 image that has the Radeon firmware.
https://cdimage.debian.org/cdimage/unofficial/non-free/cd-including-firmware/current/amd64/