`dd` does not create bootable Ubuntu iso

You've got a file as /dev/sda not a device, so when you write to /dev/sda you're overwriting the file. With your NVMe disk this explains why writing speed is so high.

Remove the file /dev/sda, unplug and replug the USB stick. Check that /dev/sda is now a block device (first character from ls -l is b) rather than a file (first character -), like this:

brw-rw---- 1 root disk       8, 0 Nov  2 17:03 /dev/sda
brw-rw---- 1 root disk       8, 1 Nov  2 17:03 /dev/sda1

How did this happen? It's possible you first tried to write to the device before it had been plugged in, so the device node hadn't yet been created. Thereafter the presence of the file prevented the device from being created.