Turtl RethinkDB Install Issues

I am attempting to install RethinkDB on Ubuntu 18.04 LTS on Linode instance and continue to get an issue finding the correct source list information for me to get the software to install.

I am wondering if anyone has run into this issue and whether or not they have found a work around to get this software installed. I am a relative newbie but I am experimenting all the time and I want to get my Turtl instance running on my Linode sooner rather then later.

Thank you in advance.