I am quite frustrated by the lack of support for the RX 6700 XT in Ubuntu. I have spent 2 days looking through support docs and trying different versions of Ubuntu. I have tried both 20.04 and 18.4.5 with the latest amdgpu-pro installation and yet no devices show when running clinfo. Yes, I have added the user to the render and video groups. Seems as if more than a few people are having the same issue. Come on AMD, if you expect us to keep trying HARD these days to find and purchase the cards retail at least you should have drivers that work.
The new drivers coming are reported to support the new 20.4 LTS 5.8 kernel, so that should resolve things. If you're running the 5.4 kernel on 18 LTS, you should be able to install the ROCm driver. I'm also patiently awaiting support for the 5.8 kernel to fix the 6700 XT on Ubuntu.