for me LibreOffice Base required a separate installation. i first opened Ubuntu Software and typed “libreoffice” (without quotes) into the search box:
there you can see LibreOffice Base listed as the 6th option from the top. when i clicked on that option, i was taken to this page which offered to let me install Base by clicking on the “Install” button:
after i clicked on install, i also got a window that asked for my password to allow the system to install Base. the previous screen changed to show that Base was installed:
where there was previously a button that said “Install”, now there are two buttons. one says “Launch” and another that says “Remove”. if you press “Launch”, that will open Base.
if you don’t launch or open Base from that screen, you can use the windows key (on your keyboard) or the 9 dots in the bottom left-hand corner of the screen or even the word Activities in the top left-hand corner to get a search box. if you have installed Base and type “base” (also without quotes) in that search box, you should get an option that looks like this:
the purple icon with “LibreO…” underneath it should open Base.
when i initially opened it, i got the java warning mentioned above. the next time i was asked to create or open an existing database: