Cassandra Issue Connection error: ‘Unable to connect to any servers’

If you installed the apache cassandra 3.10 on ubuntu 20.4 which have the java 11 is default. You can not work with it as the apache cassandra is not worked with java 11.

You need to downgrade to java 8 in order to work with the apache cassandra.

Install Java 8

sudo apt-get install openjdk-8-jre
sudo update-alternatives --config java

You can select the java 8. You can check the java version with following command

java -version