this is b/c step 3 of the link your provided needs a bit of work ...
$ wget -qO- https://deb.torproject.org/torproject.org/A3C4F0F979CAA22CDBA8F512EE8CBC9E886DDD89.asc | gpg --dearmor | sudo tee /usr/share/keyrings/tor-archive-keyring.gpg >/dev/null
this will bring in the key correctly.
After sudo apt update
you may get a warning ... N: Skipping acquire of configured file 'main/binary-i386/Packages' as repository 'https://deb.torproject.org/torproject.org focal InRelease' doesn't support architecture 'i386'. Then you can install ...
sudo apt install deb.torproject.org-keyring
sudo apt install tor
(tested on my um20.4)
Or
as that page says, Note: The symbol # refers to running the code as root.
First become root: sudo su -
then you can follow their steps as root.
( Warning: you can mess up your system if you don't know what you're doing as root)
Note: the repo tor.list should be:
cat /etc/apt/sources.list.d/tor.list
deb [arch=amd64 signed-by=/usr/share/keyrings/tor-archive-keyring.gpg] https://deb.torproject.org/torproject.org focal main
deb-src [arch=amd64 signed-by=/usr/share/keyrings/tor-archive-keyring.gpg] https://deb.torproject.org/torproject.org focal main