Comment 3 for bug 1908147

And I think it is because the package is held:

ubuntu@juju-d6d3e6-0:~$ dpkg-query --list cassandra
Desired=Unknown/Install/Remove/Purge/Hold
| Status=Not/Inst/Conf-files/Unpacked/halF-conf/Half-inst/trig-aWait/Trig-pend
|/ Err?=(none)/Reinst-required (Status,Err: uppercase=bad)
||/ Name Version Architecture Description
+++-=============================-===================-===================-===============================================================
hi cassandra 3.11.9 all distributed storage system for structured data

While the charmhelper checks for a status of 'ii'.