If you prefer sqlplus not installed yet, install Oracle Instant Client separately, or use the SQL*Plus included in the XE installation:
[Install] WantedBy=multi-user.target
source ~/.bashrc # Start sudo /etc/init.d/oracle-xe-21c start Stop sudo /etc/init.d/oracle-xe-21c stop Status sudo /etc/init.d/oracle-xe-21c status install oracle xe 21c on ubuntu