Am 30.06.20 um 11:25 schrieb dexterd:

Hi, I followed the instructions in the official website (https://git.skewed.de/count0/graph-tool/-/wikis/installation-instructions#de...) 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 python3-graph-tool"...

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There are no packages for xenial, since it's outdated and has reached end of life.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).