Unable to install on ubuntu 16.04 LTS

Hi, I followed the instructions in the official website
to install graph-tool on ubuntu 16.04 LTS. I set DISTRIBUTION to xenial
(corresponding to ubuntu 16.04 LTS). Then I downloaded the public key as
instructed. However, when I ran apt-get update, it reported the error:
"The repository 'https://downloads.skewed.de/apt xenial Release' does not
have a Release file". The following line would not work, neither.
I wonder which DISTRIBUTION should I set? I tried the ones listed in the
website but failed when executing the "apt-get install

Thank you very much for your time and I look forward to you reply.

There are no packages for xenial, since it's outdated and has reached
end of life. You should upgrade to a supported ubuntu version (the
current LTS is 20.04, but 18.04 will work too).

I'm sorry, this is incorrect, xenial has not reached end of life. It's
just too old to compile graph-tool.