Sagemath-9.0 installed from the package is in 20.04 (soon coming release
29-4 maybe), either you wait or you can try to install sage-9.0 from
binary on the website. I tried few times to installed it but didn't succeed.
If you want to try download the binary from sagemath website
decompressed it and run it (be careful because you can't change the dir
after having it ran !
Try to run sage in a shell and if you want to use jupyter notebook you
can make a link
sudo ln -s sage /usr/bin/sage
Then you can run it from anywhere (but without sudo !)