LibreOffice is installed by default on Linux Mint which includes LibreBase.
LibreBase can connect to existing databases like MySQL but Postgres is not one of the options listed.
To add Postgres to the list of existing database types you need to install the postgres driver. In Linux Mint this can be done via the Software Manager.
Launch the Software Manager, then search for
Open the entry by double-clicking on
Click on the button labelled
LibreBase you should now see
PostgreSQL as an option.