I bet you tried every suggestion on the Worldwide Web too. Flatpak did not work for me. I think I have tried every known way suggested now.
I would take it off completely. Assuming you use a terminal then you can purge everything LibreOffice. Your files remain. Then use the terminal to reinstall. You will only get 5.2.
Chromebooks seem unable to cope with 6. I think the Chromebook makers thought everyone would be happy to use Google docs instead. Google docs is not great for formatting. We still need LibreOffice. Even with 5 I have had to reinstall several times. There are those who say they managed to install and open LibreOffice 6 from deb. files, but nothing ever worked for me. I have not tried LibreOffice on a stick, but apparently, you can download a version onto a memory stick and perhaps that will work. Someone here mentioned that yesterday.
Anyway, to get 5.2 open a terminal.
sudo apt-get remove --purge libreoffice*
sudo apt-get clean
sudo apt-get autoremove
sudo apt-get install libreoffice
For now, this seems to be as good as we can get. However, there are plenty of add-ons to beef it up with. I live in hopes after every update. The other way is to install a Linux derivative alongside what you get on a Chromebook. I did this for a while, but it was not as good as working in Ubuntu on my laptop. It was also a pain changing OS all the time. (Then one night the cat sat on the keyboard for an entire night and deleted an entire OS!) I did get version 6. The interface was not great. Also, few people are brave enough to mess with their hardware like that and you void the guarantee.
The problem seems connected to Crostini. There seems to be a lot online about Crostini containers not loading and we need them to make any Linux application work -Libre Office only works in the Linux Beta of a Chromebook. LibreOffice currently only opens for me after reinstalling it and the terminal works to reinstall then refuses to do anything else. This is not a problem limited to LibreOffice. I suspect it will sort on an update. I am tempted to install a Linux OS alongside just to get LibreOffice 6 and a stable experience!