tar -Jxvf amdgpu-pro-18.30-633530.tar.xz
xz: (stdin): File format not recognized
tar: Child returned status 1
tar: Error is not recoverable: exiting now
Your probably not going to find much help here. AMD official drivers for open source pretty much stopped at 2015 (IoM) Edition
You can head to open source community for Xorg found HERE
For alternative find HERE
If your still under a frsh install of Ubuntu
It was developed by the X11 community, mostly centered around Novell and AMD, with the free documentation provided by AMD.
Follow the Alternative commands and report your errors to were Xorg directs it.
This really isn't the OpenSource Support forum, but hope my info helps ya out
Might be a regional issue with the problem of downloading that AMDGPU PRO driver. I just downloaded it a few minutes ago without any problems.
If you want you can download the extracted AMDGPU PRO File from this free Simple File hosting website by just clicking on the link where I uploaded the file. It will be there for 30 days only : https://nofile.io/f/mcSWW8mg9pf/amdgpu-pro-18.30-633530.tar
I extracted the AMDGPU PRO so that the extension will change from TAR.XZ to TAR. Some Linux Users in the past were having problems using the TAR.XZ format.
Thank you very much downloading!!!
Retrieving data ...