I will follow this step for make sure it can work correctly
This video use Knowage 6.2.1 on Ubuntu 18 and Use Oracle JDK 8 and MySQL Server 5.7
Step 1. Install Java / Step 2. Configure System Variables
sudo apt install software-properties-common sudo add-apt-repository ppa:webupd8team/java sudo apt update sudo apt install oracle-java8-installer sudo apt install oracle-java8-set-default # Setup JAVA_HOME export JAVA_HOME=/usr/lib/jvm/java-8-oracle export PATH=$JAVA_HOME/bin:$PATH # Verify JDK is Installed java -version # java version "1.8.0_201" # Java(TM) SE Runtime Environment (build 1.8.0_201-b09) # Java HotSpot(TM) 64-Bit Server VM (build 25.201-b09, mixed mode)Reference: Command [add-apt-repository] Missing | Install Java 8 | Setup System Variables