Search for the snap:
$ snap find cassandra
Name Version Developer Notes Summary
cassandra 3.7 ev - Cassandra distributed database
Install the snap:
$ sudo snap install cassandra
[sudo] password for elopio:
cassandra (stable) 3.7 from 'ev' installed
Check the status:
$ cassandra.nodetool status
Does the following resolve the issue for you?
sudo snap connect cassandra:mount-observe ubuntu-core:mount-observe
sudo snap connect cassandra:process-control ubuntu-core:process-control
sudo systemctl restart snap.cassandra.cassandra.service