as i also use (k)ubuntu here my quick install info.
Open terminal and enter (add sudo if needed):
your java-version:
java -version
Install java:
apt install openjdk-8-jre
apt install openjfx
Download jabref from jabref.org
Create Install directory:
mkdir /opt/jabref
mv downloaded jabref to install directory:
mv /home/yourusername/Downloads/JabRefdownloadname.jar /opt/jabref/jabref.jar
Create starter by openeing editor - e.g. kate:
SUDO_EDITOR=kate sudoedit /opt/jabref/jabref.sh
File Content:
#!/bin/bash
java -jar /opt/jabref/jabref.jar &
Terminal again:
make executable:
chmod +x /opt/jabref/jabref.sh
Start with:
java -jar /opt/jabref/jabref.jar
and/or create Symlink:
ln -s /opt/jabref/jabref.sh /usr/local/bin/jabref
this worked for me.