Installing Java or Java runtime

Bring up a Terminal window (using Spotlight or going to Applications, Utilities folder, Terminal) and type in:

java -version

Does it come up with a response? For example, I use Java 1.8 build 191 and get the following:

I strongly recommend that you actually take the step of installing the Java JDK. The JRE version is **** and often won't update your system variables properly (like the Java Home folder).

For example, as an additional test you can type in:

echo $JAVA_HOME

Does it list a particularly folder? Such as:

Anyway, all this aside, simply install the Java JDK.

In fact, I just checked... LibreOffice says the JDK is required for macOS 10.10 and later (https://www.libreoffice.org/get-help/system-requirements/) so that definitely settles it!

Get the JDK here:

https://www.oracle.com/java/technologies/javase/javase-jdk8-downloads.html

You have to make a free Oracle account to download the JDK. I linked you to the 1.8 version which is what I recommend. It's also what Oracle recommends too for most people, the latest build is 271. I should probably update mine. :)

So you have multiple Java's installed and your macOS default (as shown via terminal) is Java 15 which is definitely not recommended. It's possible that even though your LibreOffice is selected to use Java 8, it's still not doing things properly and I suggest changing your macOS's Java itself to be 1.8 also by changing the Java Home environmental variable.

You should be able to do this:

export JAVA_HOME=$(/usr/libexec/java_home -v 1.8)

And then do

java -version

to see if it now shows the 1.8 build 271 as the default.

If that doesn't work you can go here to delete the Java 15 via Finder, Go, Go to Folder, and put in:

/Library/Java/JavaVirtualMachines/

I'm assuming that's where it is for you, but to not assume you can type in the following to see where your current java home is set to:

/usr/libexec/java_home -v 1.15

And that's where it should show all your Java JDKs installed and you get get rid of the Java 15. For example, here's a screenshot that shows what Javas I have and where you can delete the 15 (presumably):

Did you specifically delete the LibreOffice support files in the ~/Application Support folder before reinstalling?

Did you try my suggestion of testing this in a new macOS user account as well? (fyi, you won't need to reinstall Java there as Java is system-wide, just the LibreOffice)