In order for Jupyter to start the Cadabra kernel, it has to be able to find the cadabra2 and cadabra2_jupyter modules. If you are using a binary package, these things will be installed in a path searched by the system python, but if you built from source, they may not be.
Typically, if you configure with cmake .. with no further options, the installation will be into /usr/local/lib/python3.7/site-packages (possibly with a different python version number). You need to add this to your PYTHONPATH, e.g. using