did you go with steps in their official installation guide? docs.liquibase.com/start/install/liquibase-linux.html they have Troubleshooting part as well: docs.liquibase.com/start/install/liquibase-installation-troubleshooting.html
liquibase installation should come with postgresql.jar driver in liquibase/internal/lib if it's not there, try to download postgres driver from: jdbc.postgresql.org/download/ and put it into liquibase/lib folder. Also check your liquibase.properties file in your project. You can set "classpath" key to point to database jar - maybe it points to non existing temporary folder?
