Please note: Paratext 8 is available without following the Beta instructions.
Also just in case you are not aware, Wasta Linux is an Ubuntu derivative that makes Paratext and SIL software easily available.
https://sites.google.com/site/wastalinux/home/download
I generally recommend this as it is the most used Linux platform for running Paratext.
Now back to your question:
Sometimes it can take a little while for Paratext to appear in the Ubuntu software center.
one option would be to just run:
sudo apt install paratext-8.0 from a terminal.
Then run paratext 8 via the system menu, or type paratext8 in a terminal.
