For any problem that involves a Cadabra kernel which does not want to start, please send the relevant error lines which are printed to the terminal. So start the sage notebook using
./sage -notebook
from a terminal, and copy-paste the output into an email to info@cadabra.science
.
I have just tried the above with the current Cadabra on an Ubuntu 18.04 system with Sage 9.2, and it works, so it is definitely possible.