Installing on Ubuntu 21.04

I second that, I used RUFUS 3.13 to install LinuxCNC 2.8.0 it worked well for an HP elitedesk 800 g1 (i5 8meg ram). Jitter around 42000ns using pre-emptrt.

I confirm the existence of the problem - Same problem with Lenovo 6072-CTO computer - (cannot see CD / DVD)
Pendrive made in LinuxLive USB Creator (LiLi) (I've always done this and old versions worked)
Only making the Pendriwa in Rufus 3.13 helped

Rufus is a Wiondows program and (as it warns you) may not contain all the files you need for a Linux install - as I found out. If you already have a Linux system, use the USB Media Creator which is part of most Distis (I have Ubuntu)
as
But don't ask me why they package it a pretty outdated Debian .iso
It would have been much better as a Ubuntu Linux Snap in the Store

..and who uses DVD drives today? I plopped a SSD in a Caddy in there YEARS ago. Get up to date, guys!!

forum.linuxcnc.org/9-installing-linuxcnc...-nearly-easy-install
Current enough?

.iso 2.8 Buster downloaded today if thats what you mean.
if you mean Rufus, also latest version